Split equation system assemble method

[IvarStefansson]

Proposed changes

1. Deletes redundant assemble_subsystem method from EquationSystem. Same functionality is now covered by assemble.
2. Add evaluate_jacobian parameter to assemble.
3. Resurrect test_equation_system, which was deleted by mistake.

Types of changes

What types of changes does this PR introduce to PorePy?
Put an x in the boxes that apply.

• Minor change (e.g., dependency bumps, broken links).
• Bugfix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• Testing (contribution related to testing of existing or new functionality).
• Documentation (contribution related to adding, improving, or fixing documentation).
• Maintenance (e.g., improve logic and performance, remove obsolete code).
• Other:

Checklist

Put an x in the boxes that apply or explain briefly why the box is not relevant.

• The documentation is up-to-date.
• Static typing is included in the update.
• This PR does not duplicate existing functionality.
• The update is covered by the test suite (including tests added in the PR).
• If new skipped tests have been introduced in this PR, pytest was run with the --run-skipped flag.

[IvarStefansson]

@keileg, could you please have an initial look.

[keileg]

I pushed a few minor changes, including one which should take care of the remaining typing issue.

I have no further comments, so if you are happy with my changes, I can do a quick formal review before we do the merge.