GeoModelCore/GeoModelIO clang-tidy fixes

GeoModelCore/GeoModelIO clang-tidy fixes

Relates to [#109 (closed)].

These are the ones that can be auto-applied by the tooling so see

/afs/cern.ch/user/c/christos/public/GeoModel_static_check/clang_tidy_log 

for the "reasoning"

GeoModelCore/GeoGenericFunctions/GeoGenericFunctions/Power.h

@@ -48,8 +48,8 @@ namespace GeoGenfun {
     // It is illegal to assign a fixed constant
     const Power & operator=(const Power &right);
 
-    double _doublePower; // power (as a double)
-    int    _intPower;    // power (as an integer)
+    double _doublePower{}; // power (as a double)
+    int    _intPower{};    // power (as an integer)
     bool   _asInteger;   // flag:  object constructed with integer argument
 
   };