METK-163: Tile key support

[mjg41]

Description

Following the approval of the addition of the tile, tileattribute and tilescheme keys in codex MARS - 006:
MARS-006-Land-Surface-Tile-Data.md

Comments and modifications very much welcome before merge!

FYI @danovaro , @simondsmart

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[danovaro]

minor changes requested, which block language parsing

[mjg41]

Thank you very much @danovaro for the review! With the latest commit I should have resolved your comments.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.71%. Comparing base (6b8bf0a) to head (1af78cf).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #198   +/-   ##
========================================
  Coverage    61.71%   61.71%           
========================================
  Files          303      303           
  Lines        11789    11789           
  Branches      1049     1049           
========================================
  Hits          7276     7276           
  Misses        4513     4513           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.