Solve little error in resample_in_space

[konstntokas]

This PR is related to #1100 and corrects a little error in resample_in_space.

If multiple data variables are in the dataset, we need to collect first all grid_mappings and subsequently deleting them. Otherwise, if two data variables have the same grid_mapping, deleting them in a for-loop causes an error, since the grid_mapping variable has already been deleted.

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.59%. Comparing base (f1654b7) to head (ee81a04).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1108      +/-   ##
==========================================
- Coverage   89.60%   89.59%   -0.01%     
==========================================
  Files         277      277              
  Lines       21852    21854       +2     
==========================================
  Hits        19581    19581              
- Misses       2271     2273       +2     

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