ENH: add `spatial_ref` with `pyproj` when georeferencing, add/adapt methods/tests

[kmuehlbauer]

This adds spatial_ref to the datasets when georeferencing. That way users can easily wrap this into cartopy and use this for georeferenced plotting.

• tests added/adapted
• added example to plot-ppi.ipynb notebook
• takes site coordinates into account (altitude) for georeferencing
• site coordinates are in default model Dataset (Locarno Monti, MeteoSvizzera coords, birthplace of xradar)

[kmuehlbauer]

@mgrover1 I've refactored the naming per your suggestion.

[codecov[bot]]

Codecov Report

Merging #87 (87537e7) into main (3435b08) will decrease coverage by 0.25%. The diff coverage is 75.40%.

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
- Coverage   88.00%   87.75%   -0.25%     
==========================================
  Files          18       19       +1     
  Lines        3227     3284      +57     
==========================================
+ Hits         2840     2882      +42     
- Misses        387      402      +15     

Flag	Coverage Δ
unittests	87.75% <75.40%> (-0.25%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
xradar/accessors.py	59.32% <37.50%> (-8.86%)	:arrow_down:
xradar/georeference/projection.py	82.75% <82.75%> (ø)
xradar/georeference/__init__.py	100.00% <100.00%> (ø)
xradar/georeference/transforms.py	100.00% <100.00%> (ø)
xradar/model.py	95.50% <100.00%> (+0.07%)	:arrow_up:

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[kmuehlbauer]

@mgrover1 I'm ironing out the CI/doc issues. We can let this sit another while. No rush for now to get this in.

[kmuehlbauer]

This is the direct link to the docs with a plotting example:

https://openradar-docs--87.org.readthedocs.build/projects/xradar/en/87/notebooks/plot-ppi.html#Plot-PPI-on-map-with-cartopy

[kmuehlbauer]

@mgrover1 Heads-up, I've found some problems with the docs which are slightly related to this PR and I'll add that here. I'll also add some more code to model.py for the sitecoords.

[kmuehlbauer]

I've also found an issue that get_x_y_z didn't take the radar site altitude into account, I've fixed this too.

[kmuehlbauer]

@mgrover1 I'll merge tomorrow, if no-one beats me to it.

[kmuehlbauer]

I've rearranged/squashed the commits into two for rebase-merge. Getting this when CI is green.