Misc Raster Utilities

This merge adds a few utilities for working with raster datasets

1. Data/NoData mask properties, and
2. Raster.from_points, which builds a raster from a set of point features, and
3. Raster(..., isbool=True), which indicates that a loaded raster should be treated as a boolean array

Closes #76 (closed), #77 (closed)

Notes for Reviewer

The key changes are in the raster.py module, as follows:

• Lines 279-285: Added properties for data/nodata masks
• Lines 539-542: Added code to optionally convert raster to boolean array
• The docstrings of init, from_file, from_rasterio, from_pysheds, and from_array are updated to explain the "isbool" option
• Lines 1018-1113: Added the from_points method

The new from_points command is modelled on the existing from_polygons command. I generalized the backend to support both commands, so the diffs will look large, but the only real change is the addition of the _validate_point function (lines 858-881).