Fix BPVCORRM functor

This MR fix the bug in the BPVCORRM functor (see #216 (closed)). In particular, when computing the direction of flight, the decay vertex position is taken using Sel::Utils::endVertexPos( composite ).

P:S.: Initially I tried to use the ->position() method on decay_vertex but a compilation error was raised.

I attach the full log.txt for details, but the problem seems to be:

/afs/cern.ch/work/g/gtuci/private/Moore_fixCorrM/stack/Rec/InstallArea/x86_64_v2-centos7-gcc10-opt/include/Functors/Core.h:384:19:   required from here
/afs/cern.ch/work/g/gtuci/private/Moore_fixCorrM/stack/Rec/InstallArea/x86_64_v2-centos7-gcc10-opt/include/Functors/Composite.h:206:22: error: base operand of ‘->’ has non-pointer type ‘const LHCb::LinAlg::Vec<SIMDWrapper::scalar::float_v, 3>’

closes #216 (closed)

cc @lpica @sklaver @mvesteri