User guide / documentation

This merge requests adds documentation to pfdf. The 3 main components of the docs are:

• A user guide introducing the key routines in pfdf
• Tutorials for specific use cases, and
• Reference API

Reviewers can find the proposed documentation here: https://ghsc.code-pages.usgs.gov/users/jking/pfdf

This merge also includes a few misc improvements. As follows:

• Fixes a Raster clipping bug,
• Improves broadcasting options in the s17 and g14 modules,
• Adds windowed raster reading
• Add the "in_mask" and "in_perimeter" methods to Segments

Closes #78 (closed), #80 (closed)