Merge pull request #6 from astrofrog/refactor-geometry

Refactor geometry
Merge pull request #5 from keflavich/master

Add WCS-based PV extraction
Refactor using Paths
Removed ``interpolation`` option, use ``order=0`` for nearest-neighbor interpolation
Small function name change
Renaming thick to poly
Tweaks to imports
Minor fixes
Improve description of Path class
Use xy in Path instead of separate x and y to ensure equal lengths of x and y
Set respect_nan to True by default
Check type and value of order
Clarify ``respect_nan``
Merge pull request #8 from astrofrog/oo-refactor

Refactor to define and use Path