Add option to set AA substitution model for raxml

[iimog]

The AA substitution model for the partitions is currently hardcoded to WAG. This choice is a little arbitrary so it should be possible for the user to change one of the other valid options in raxml 8:
DAYHOFF, DCMUT, JTT, MTREV, WAG, RTREV, CPREV, VT, BLOSUM62, MTMAM, LG,
MTART, MTZOA, PMB, HIVB, HIVW, JTTDCMUT, FLU, STMTREV, DUMMY, DUMMY2, AUTO,
LG4M, LG4X, PROT_FILE, GTR_UNLINKED, GTR

Also using AUTO as default might be a more sensible choice.
If we change this default behaviour the minor version number should be increased by one.

[iimog]

Needs to be added to the GUI as well