Addition of raster functionality

[kbonney]

Summary

This PR adds the function sample_raster to the wntr.gis module. The function samples a raster file at the coordinate points of the provided geodataframe (e.g., junctions from WaterNetworkGIS). This PR only extends the existing API and does not modify it (ie, the inclusion of the PR merits a minor version change at most).

Tests and documentation

One test is added to verify that the raster is correctly sampled. A docstring is included in the function definition. Currently no additional documentation is included, but there could be a subsection added to the Geospatial capabilities user manual page to discuss and exemplify usage of the new function.

Acknowledgement

By contributing to this software project, I acknowledge that I have reviewed the software quality assurance guidelines and that my contributions are submitted under the Revised BSD License.