FileFormats.MPS: support INDICATOR

[matbesancon]

It seems most MILP solvers include INDICATOR as an extension of the MPS file format:
http://www-eio.upc.edu/lceio/manuals/cplex-11/html/reffileformatscplex/reffileformatscplex15.html
https://www.gurobi.com/documentation/9.1/refman/mps_format.html
(SCIP also)

[odow]

Format seems to be the same among solvers: