AFP,ALFA clang-tidy and cppcheck related fixes

Christos Anastopoulosrequested to merge
ATLAS-EGamma/athena:AFP_clang_cppcheck into master

AFP,ALFA clang-tidy and cppcheck related fixes

The main ones relate to

  • use nullptr
  • pass by const reference
  • initialize variables
  • use empty()
  • prefer prefix
  • use c++ style headers e.g. cmath rather than math.hetc

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