Rename dimension parameter to group_by.

[aaronweeden]

WORK IN PROGRESS

Description

This PR renames the dimension parameter of the get_data() method to group_by.

The dimension parameter will still work but is given a deprecation warning.

Motivation and Context

The goal is to have a parameter name that is more accurate and clear. Dimensions can be used for both grouping and filtering. In this case, the dimension is used for grouping.

Tests performed

Types of changes

• Refactoring / documentation update (non-breaking change which does not change functionality)
• Bug fix (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 change)
• Release preparation

Checklist:

• CHANGELOG.md has been updated
• The milestone is set correctly on the pull request
• The appropriate labels have been added to the pull request
• Running the automated tests (see docs/developing.md) produces no errors
• Updates have been made to the xdmod-notebooks repository as necessary, and the notebooks all run successfully