Make data loading more efficient by opening each source file once only

[davidorme]

Description

This PR updates the core.Date.load_data_config method to group data variables by their source file. The load_to_dataarray function and readers in core.readers are then also updated to take a list of variable names rather than just a single variable name and return a dictionary of loaded data arrays keyed by variable name.

The tests and docs are also updated to the new syntax and a few extra tests of multiple loading are included.

Fixes #711

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.73%. Comparing base (97ee94b) to head (52c44aa).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #712   +/-   ##
========================================
  Coverage    94.72%   94.73%           
========================================
  Files           73       73           
  Lines         4930     4939    +9     
========================================
+ Hits          4670     4679    +9     
  Misses         260      260           

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