Fix/issue 61 variable coord selection

[Jayant-kernel]

Describe your changes

This PR completely decouples variable sampling coordinates during extraction in create_dataset. Previously, extracting variables with disjoint coordinates (like u at altitudes [100, 50] vs v at [100, 75]) would mistakenly align and broadcast their Union dimensions if appended directly back onto a shared xr.Dataset.
I have resolved this by substituting the intermediate compilation ds = xr.Dataset() into a native Python dictionary ds = {} until the final combination phase. This completely negates implicit NaN expansion.
No new external dependencies were introduced, and ops/stacking.py + ops/mapping.py now support unmerged dictionaries natively.

Issue Link

Closes #61

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

Checklist before requesting a review

• My branch is up-to-date with the target branch - if not update your fork with the changes from the target branch (use pull with --rebase option if possible).
• I have performed a self-review of my code
• For any new/modified functions/classes I have added docstrings that clearly describe its purpose, expected inputs and returned values
• I have placed in-line comments to clarify the intent of any hard-to-understand passages of my code
• I have updated the documentation to cover introduced code changes
• I have added tests that prove my fix is effective or that my feature works
• I have given the PR a name that clearly describes the change, written in imperative form (context).
• I have requested a reviewer and an assignee (assignee is responsible for merging)

Checklist for reviewers

Each PR comes with its own improvements and flaws. The reviewer should check the following:

• the code is readable
• the code is well tested
• the code is documented (including return types and parameters)
• the code is easy to maintain

Author checklist after completed review

• I have added a line to the CHANGELOG describing this change, in a section
  reflecting type of change (add section where missing):
  • added: when you have added new functionality
  • changed: when default behaviour of the code has been changed
  • fixes: when your contribution fixes a bug

Checklist for assignee

• PR is up to date with the base branch
• the tests pass
• author has added an entry to the changelog (and designated the change as added, changed or fixed)
• Once the PR is ready to be merged, squash commits and merge the PR.

[leifdenby]

In general this is looking good, but I think you included changes related to #96. Please remove these and request a review from me :)

[leifdenby]

I don't think this relates to the issue you are fixing here, right?