Parsing objective sense in MPS file

[stephenjmaher]

Fixes #655

This PR makes it possible to set the objective sense in an MPS file. Currently, the objective sense in the MPS file is ignored. The sense needs to be supplied to fromMPS as a parameter.

This change does the following:

• the sense parameter in LpProblem.fromMPS now defaults to None.
• the readMPS methods checks for the header OBJSENSE in the MPS file. If the header is encounter there are two options:

  OBJSENSE   MAX

  or

  OBJSENSE
  MAX

  both options are handled in the readMPS method.

• If no objective sense is specified in the MPS file, then sense defaults to LpMinimize.
• If an objective sense is passed to LpProblem.fromMPS, then this overrides the sense that is specified in the MPS file.

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Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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