adding split_multi_geometries() and a warning for empty gdfs

[nrweir]

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Description

Per @chrieke's points in #187 (thanks again!):

• implementing split_multi_geometries where I forgot to in vector_tiler.VectorTiler.tile_generator()
• adding a warning when an empty gdf is made with sol.utils.core._check_gdf()

Eventually I'll clean up the docs per your suggestion, but not yet. I'll open another issue to track it.

Fixes #187

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected - these changes will not be merged until major releases!)

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• My code follows PEP8
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[codecov]

Codecov Report

Merging #189 into dev will increase coverage by 0.01%.
The diff coverage is 70%.

@@            Coverage Diff             @@
##              dev     #189      +/-   ##
==========================================
+ Coverage   71.31%   71.33%   +0.01%     
==========================================
  Files          65       65              
  Lines        4058     4064       +6     
==========================================
+ Hits         2894     2899       +5     
- Misses       1164     1165       +1

Impacted Files	Coverage Δ
solaris/tile/vector_tile.py	84.61% <100%> (+0.4%)	⬆️
solaris/utils/geo.py	71.53% <100%> (+0.21%)	⬆️
solaris/utils/core.py	64.47% <25%> (-0.4%)	⬇️

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