Reorganization of Counters headers

• renamed GaudiKernel/Counters.h to Gaudi/Accumulators.h, as by the convention that file names should match the namespaces
• moved StatEntity from Gaudi/Accumulators.h to GaudiKernel/StatEntity.h
• fixed Gaudi/Chrono/Counters.h and Gaudi/Accumulators.h inclusion order issue (using forward declarations)
• move counters unit tests to a proper unit test

Closes #99 (closed)

GaudiExamples/src/CounterEx/CountersUnitTest.cpp

-#include "Gaudi/Chrono/Counters.h"
-#include "GaudiKernel/Counters.h"
-
-#include <chrono>
-#include <iostream>
-
-using namespace Gaudi::Accumulators;
-
-int main() {
-
-  // Testing Buffer moving
-  {
-    Counter<double, atomicity::full> c;
-    c += 3.5;
-    c += 1.2;
-    {
-      auto buf = c.buffer();
-      buf += 7.2; // buf1 internal count = 1
-      decltype( buf ) buf2( std::move( buf ) );
-      buf2 += 3.5; // buf3 internal count = 2
-      decltype( buf ) buf3 = std::move( buf2 );
-      buf3 += 3.7; // buf3 internal count = 3
-    }
-    std::cout << c << std::endl; // should output 5
-  }
-
-  // Testing counter resets
-  {
-    SigmaCounter<> sig;
-    sig += 3;
-    sig += 5;
-    sig += 6;
-    std::cout << sig << std::endl;
-    sig.reset();
-    std::cout << sig << std::endl;
-  }
-
-  // Testing AveragingCounter
-  {
-    AveragingCounter<> avg;
-    avg += 3;
-    avg += 5;
-    avg += 6;
-    std::cout << avg << std::endl;
-  }
-
-  // Testing integer AveragingCounter
-  {
-    AveragingCounter<int> avg;
-    avg += 3;
-    avg += 5;
-    avg += 6;
-    std::cout << avg << std::endl;
-
-    static_assert( std::is_integral_v<decltype( avg.mean<int>() )>, "Integral return type expected" );
-    static_assert( std::is_floating_point_v<decltype( avg.mean() )>, "Floating point return type expected" );
-  }
-
-  // Testing SigmaCounter
-  {
-    SigmaCounter<> sig;
-    sig += 3;
-    sig += 5;
-    sig += 6;
-    std::cout << sig << std::endl;
-  }
-
-  // Testing AveragingCounter with buffering
-  {
-    AveragingCounter<float, atomicity::full> avg2;
-    {
-      auto bufAvg = avg2.buffer();
-      for ( int i = 0; i < 1000; i++ ) bufAvg += i;
-    }
-    std::cout << avg2 << std::endl;
-  }
-
-  // Testing Binomialcounter
-  {
-    BinomialCounter<> bin;
-    bin += false;
-    bin += true;
-    bin += true;
-    bin += false;
-    bin += false;
-    std::cout << bin << std::endl;
-  }
-
-  // Testing Binomialcounter with buffers
-  {
-    BinomialCounter<> bin;
-    {
-      auto bufBin = bin.buffer();
-      bufBin += false;
-      bufBin += true;
-      bufBin += true;
-      bufBin += false;
-      bufBin += false;
-    }
-    std::cout << bin << std::endl;
-  }
-
-  // Testing bulk increment iwth Binomialcounter
-  {
-
-    BinomialCounter<> bin;
-    // bin += { .nPass = 4, .nTotal = 10 }; // C++20 only...
-    bin += {4, 10};
-    bin += {1, 9};
-    bin += {0, 1};
-    std::cout << bin << std::endl;
-  }
-
-  // Testing StatEntity, the backward compatible counter
-  {
-    StatEntity se;
-    se += 3;
-    se += 5;
-    se += 6;
-    std::cout << se << std::endl;
-  }
-
-  // Testing StatEntity, the backward compatible counter with binomial usage
-  {
-    StatEntity sb;
-    sb += 0;
-    sb += 1;
-    sb += 1;
-    sb += 0;
-    sb += 0;
-    std::cout << sb << std::endl;
-  }
-
-  // Testing StatEntity, setting directly values
-  {
-    StatEntity sb{3, 14, 70, 3, 6};
-    std::cout << sb << std::endl;
-  }
-
-  // Testing Counters of std::chrono::duration
-  {
-    typedef std::chrono::milliseconds  Unit;
-    StatCounter<Unit, atomicity::full> c;
-    c += Unit( 5 );
-    c += Unit( 3 );
-    c += std::chrono::seconds( 1 ); // mixing with larger units is OK
-    std::cout << c << std::endl;
-    // Check compilation of all stat methods:
-    static_assert( std::is_same_v<decltype( c.mean() ), Unit>, "Return type should be Unit" );
-    static_assert( std::is_same_v<decltype( c.standard_deviation() ), Unit>, "Return type should be Unit" );
-    static_assert( std::is_same_v<decltype( c.unbiased_sample_variance() ), Unit>, "Return type should be Unit" );
-    static_assert( std::is_same_v<decltype( c.biased_sample_variance() ), Unit>, "Return type should be Unit" );
-    static_assert( std::is_same_v<decltype( c.meanErr() ), Unit>, "Return type should be Unit" );
-    static_assert( std::is_same_v<decltype( c.min() ), Unit>, "Return type should be Unit" );
-    static_assert( std::is_same_v<decltype( c.max() ), Unit>, "Return type should be Unit" );
-  }
-}