2.7.4. extras_mod.f90 options

The extras/ directory, and the related extras_mod.f90 template file provide optional extra features, which are only linked into the executable if explicitly selected.

In practice, this is done by copying the extras/extras_mod.f90 template file to the experiments/whatever/ directory, uncommenting lines related to the selected extra features, and adding a line listing those features in experiments/whatever/Makefile.