Avoid LOCALE in MPS writing

[sebhoerl]

We just noticed that String.format in MpsAssignmentWriter is dependent on the system locale. So on a French system, decimal numbers will be written out with a , instead of . as decimal separator. This, in turn, lets GLPK or Cbc fail because it is not a valid problem. Need to replace the calls to String.format by String.format(Locale.ENGLISH, ...).