FEAT: Add offset to hydrostatic and lithostatics BCs

[jwboth]

Proposed changes

Addresses #1698. Introduces the possibility to add offset to hydrostatic pressure and lithostatic stress BCs. When using lithostatic multipliers unequal 1, the offset is typically different for each dimension such that the lithostatic stress offset needs to be a vector.

Types of changes

What types of changes does this PR introduce to PorePy?
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• 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

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• 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]

Some minor comments on documentation.