Mega patch to modernise build system

OK, so here's a number of changes - mainly to build system - to make it easier to use CRMC shared libraries in third party applications.

The changes are

• Use CMAKE configure_file macro instead of hand-crafted replacements This effects all files ending in -HEPMCTEMPLATE or -HEPMC-TEMPLATE

• In case ROOT is enable, do not set -std=c++X - ROOT will supply that option

• Sub-module CMakeLists.txt search for files not only in current source directory but also current binary directory. This is needed for out-of-source builds (don't eat where you ... - you get the idea) so that CMAKE can find the generated files (from configure_file)

• Enforce that only one DPMJet model version can be built

• Enforce, in case of static linking, that PhoJet and DPMJet cannot be built together

• In case of static linking, check if both C++ and Fortran compilers support the flag -mcmodel=medium and if so, pass that flag. This is needed - otherwise we might get the error

  relocation truncated to fit: R_X86_64_32S against symbol ...

  from GCC (and I think other compilers too)

• Use

  set_property(SOURCE files APPEND PROPERTY COMPILER_DEFINITIONS ...)

  instead of setting COMPILER_FLAGS in a two step fashion.

• Some other minor code fixes

  • src/models.F Preprocessor guard around PHO_PHIST expanded to take into account if PhoJet is being built
  • src/dpmjet/3.2019-1/DPMJET-19.1/src/phojet/PHO_SETPCOMB.f - fix bug. Trying to format a CHARACTER array as REAL.

Some things to consider for the future:

• Perhaps EPOS should be treated like any other model
• Perhaps define own transform from HEPEVT to HepMC3::GenEvent procdure so that the size of the HEPEVT common block can be directly defined here
• ROOT output should really use the "standard" TGenerator output - that is, a TClonesArray of TParticle objects. Additional information can still be stored in the output tree (separate branches)