feat: configure supported models at build time

[wpbonelli]

A potential approach for #1491

• Configure supported models & exchanges at build time via meson options
  • Move model sources to independent folders for easier filtering
  • Move model/exg creation to new modules, generated from templates with fypp
  • rename ConnectionBuilder -> ConnectionFactory, move to top level, generate from template
  • rename SimulationCreate -> Simulation
• Generate IDM files with meson
  • use configure_file() before compiling
  • definition files still created with dfn2f90.py, moved to src/IDM
  • model input file and consolidated selector file generated with fypp in src/
  • generic framework files still in src/Utilities/IDM
  • dfn2f90.py accepts input DFNs via config file, default utils/idmloader/dfns.txt
  • dfn2f90.py accepts an output directory path
  • rename files, modules and routines for basename templating
    • consistent DFN & IDM file naming
    • remove indices from file names

Considerations

• Template processing is also possible with pure python if not worth adding fypp
• To keep support for make/MSVS, configure_file() is used instead of generator() and IDM/templated files are still under version control. Developers not using meson can generate code manually before build time, like is currently done for IDM
• If support for make/MSVS is ever dropped and meson becomes the standard build tool, could switch to generator(), preprocess at build time and remove generated files from versioning, but debugging may become trickier