Resolve "Add input of boundary segmentation"

What does this MR do?

Revamp GridCreator and GridWrapper to support the input and load-balancing of boundary segmentation.

Implementation:

• Make GridCreator build the grid upon construction.
• Have GridCreator use GridMapper only internally.
• Ship grid, and element and boundary index maps through GridCreator.

Usage:

• Add new config key section [grid.mapping].
• Set global indices as default values for mapping datasets.

Docs:

• Add separate user doc page for grid creation and mapping
• Add doxygen module for grid handling

Tests:

• Test correct mapping in test-grid-creation.
• Adjust system and unit tests to new config keys

Is there something that needs to be double checked?

At least not yet.

Can this MR be accepted?

• Implemented features
• Updated unit test to check feature
• Added docs
• Pipeline passing
• Added entry to CHANGELOG.md

Related issues

Closes #123 (closed)

Meta-issue #119 (closed)