Unification of variable names throughout the code base

[keileg]

Proposed changes

This PR attempts to unify variable names used in the code base and tests to ease future maintenance. The following conventions are introduced:

• An object representing a multiphysics model should be called model
• An object of class EquationSystem should be called equation_system
• A list of subdomains should be called subdomains. When iterating over the list, the iteration variable will normally be sd
• A list of interfaces should be called interfaces. When iterating over the list, the iteration variable will normally be intf
• A single boundary grid should be denoted bg
• The domain boundary, returned by the method domain_boundary_sides should be called domain_sides

For most of these, there are exceptions that are somewhat vaguely defined (hence not written down here). Still, we will try to enforce these rules in future code reviews.

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.

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