Warm Starts for HiGHS_CMD

[aphi]

PR changes:

• In highs_api.py adds the setting of warm starts for the HiGHS_CMD solver. This involves writesol implementing the HiGHS raw solution format. This format doesn't have it's own documentation but rather the HiGHS devs recommend observing the format from what HiGHS outputs for its own solutions (see: When solving the MIP problem, can I set an initial feasible solution to HIGHS ERGO-Code/HiGHS#1284 (comment)).

• Adds HiGHS_CMD into the list of warmstart testable solvers

• Makes some unimportant typo fixes in existing code.

[aphi]

Note: The current CI failures aren't related to this PR. See Issue #714

In this PR I have commented-out COPT from the solvers used in CI until the solver has been patched, as it otherwise is blocking to all PRs on PuLP.

Resolved by COPT team with a patch.