Implement ACIS gain map

[hamogu]

Currently, MARX uses the FEF simplification for the ACIS gain, not the full gain map, because that is several orders of magnitude faster. I should investigate, if the full gain could be supported as an option for simulations that need to be as close as possible and where runtime is not a concern.

There might be already some hooks in place:
from marx-feat.h:

/* One or the other of these may be defined, but not both. */
#define MARX_HAS_ACIS_GAIN_MAP          0
#define MARX_HAS_ACIS_FEF               1

[hamogu]

It's actually implemented (just grep for "MARX_HAS_ACIS_GAIN_MAP"); but I'm not sure that implementation is complete and correct. Need to test and then maybe make a switch so that the user can select between FEF and GAIN MAP at run-time instead of me selecting it at release time by hard-coding in marx-feat.h.