Add support for Xiangqi

[mlanher]

Note these very hacky situations:

engine/3rdparty/Fairy-Stockfish/position.h

• Added class FairyBoard as friend to Position

engine/3rdparty/Fairy-Stockfish/ucioption.cpp

• Commented out option "Threads"

engine/src/uci/variants.h

• Added !!!hardcoded!!! SUBVARIANT_NB iff XIANGQI is active

Also note the following:

engine/src/environments/fairy_state/fairyinputrepresentation.cpp

• set_bits_from_bitmap inconsistent with CrazyAra as boards in data set were preprocessed in different order:
  First index equals upper left corner from view of red player

engine/3rdparty/Fairy-Stockfish/evaluate.h

• You have to set the name of the .nnue file

Finally the following dependency has been added for preprocessing of csv data sets for Xiangqi:

Pandas

[QueensGambit]

Thanks a lot for this PR.
Please have a look at the broken continuous integration:

• https://travis-ci.org/github/QueensGambit/CrazyAra/jobs/768483923

[QueensGambit]

I fixed the remaining issues.
Thanks again for your great work!