hotfix split_multi_geoms()

[nrweir]

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Description

sol.utils.geo.split_multi_geometries() was broken because it wasn't dropping multigeometries when it split them - it was simply adding each individual sub-geometry to the gdf, but preserving the "parent" multigeom. This is fixed here.

Type of change

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• [x] 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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[nrweir]

Unfortunately the travis builds are currently failing because Travis can't seem to install dev versions of projects. I'm going to merge anyway, as I'm able to complete the pip install and run all the pytests successfully.