Side effect free write functions

[Nickleaton]

Describe the new feature

In mps_lp split writeLP and writeMPS to have versions that produce a string with the contents of the file.

That way you easily write side effect free code.

Additional info

Please answer these questions before submitting your feature request.

Is your feature request related to an issue? Please include the issue number.

No

Does this feature exist in another product or project? Please provide a link.

Best practice