Fix stringent compiler warnings

Fixed various compiler warnings, some of which looked like real (albeit minor) bugs.

The compilers used were:

GCC   11.2.0: g++     -Wall -Wextra -Woverloaded-virtual -Wshadow
Clang 12.0.0: clang++ -Wall -Wextra -Woverloaded-virtual

This test can be repeated with LCG_102:

source /cvmfs/sft.cern.ch/lcg/views/LCG_102/x86_64-centos7-gcc11-opt/setup.sh
mkdir build
cd build
CXXFLAGS="-Wall -Wextra -Woverloaded-virtual -Wshadow" cmake ..
make -j4

(and similarly for x86_64-centos7-clang12-opt).

Alternatively, the stand-alone GNUmakefile (make in the repo root) has those warnings already enabled, but doesn't make all the test suite.

The particular warnings from GCC were:

-Woverloaded-virtual
-Wunused-parameter
-Wignored-qualifiers
-Wimplicit-fallthrough

After fixing those, Clang identified some more:

-Winconsistent-missing-override
-Wmismatched-tags
-Winstantiation-after-specialization

Clang identifies one remaining warning:

[ 26%] Building CXX object CMakeFiles/RooUnfold.dir/src/RooUnfoldInvert.o
src/RooFitUnfold.cxx:863:45: warning: unused parameter 'alg' ('regparam') [-Wunused-parameter]
RooAbsPdf* RooUnfoldSpec::makePdf(Algorithm alg, Double_t regparam){

but this is only activated with -DNO_WRAPPERPDF, so not trivial to fix in general.

One other change: listed new algorithms in RooUnfoldTest -h help.