Add mps support

[dthuerck]

✅ I have added the tests to cover my changes.
✅ I have updated the documentation accordingly.
✅ I have read the CONTRIBUTING document.

Summary

See issue #582 - adds support for mps and mps.gz, .lp.gz files through docplex.

Details and comments

gzip is used to extract files on the fly. I replicated the single test that uses lp files and modified it so that it uses mps files.

[coveralls]

Pull Request Test Coverage Report for Build 7339423754

• 54 of 54 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 93.099%

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Totals
Change from base Build 7144510537:	0.07%
Covered Lines:	4560
Relevant Lines:	4898

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💛 - Coveralls

[dthuerck]

Moving this back to draft as there is still a permission error with temporary files under windows.

[t-imamichi]

Since QuadraticProgram (QP) code is already big, we carefully add methods to it. It increases the maintenance cost.
qiskit_optimization has modules to separate functionality as follows and I suggest take advantage of it.

• qiskit_optimization.translators: translate between QP and other objects
• qiskit_optimization.converters: convert QP to QP in another form.

For this PR's purpose, I suggest making functions qiskit_optimization.translators.{from_mps,to_mps} in a separate file such as mps.py in qiskit_optimziation/translators.

QP has methods read_from_lp_file/write_to_lp_file before we introduce the translators. So, it might be nice to make from_lp_file to_lp_file functions to refactor and simplify QP codes in quadratic_program.py.

[dthuerck]

As discussed in #581, will replace this PR with another one.