Allow for tolerance specification in domain_boundary_sides()

[jhabriel]

Proposed changes

This small PR aims at increasing the flexibility of the domain_boundary_sides() method by allowing for specification of a tolerance instead of hardcoding a value.

In addition, documentation and typing has been updated to current standards.

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, etc).
• 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, etc).
• 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 duplicated existing functionality.
• The update is covered by the test suite (including tests added in the PR).