Fil-finder

Primarily added infrastructure improvements:
* adding testing support for python 3.7 to 3.9
* dropping support for python 3.6
* adding an action to build the distribution and wheels, and uploads to pypi

As noted in 54, the v1.6 release broke with new changes to networkx. This release fixes that issue.

Implements the new `FilFinder2D` and `Filament2D` classes for running the algorithm. The old `fil_finder_2D` is now deprecated. Also improved and expanded the suite of tests.

To see a full list of changes, see 41.

Version 2 of networkx introduced some breaking changes. These are fixed in the new release and all version of networkx should work now.

Other changes:
* fixed an issue in reading in the angular pixel scale from a header when the `PC` keywords were set.
* fixed a small bug in the failure criteria for Gaussian profile fits

Provides fixes for numpy 1.12, which requires integers for slicing arrays.

threshold_adaptive` in skimage was slightly altered, causing issues for FilFinder.

Other notable changes:
- `radial_profile` was updated to be more robust for removing variations due to overlap with different objects (`auto_cut`)
- Tools for finding the width along the main filament branch (profiles across each skeleton pixel) were introduced in `fil_finder.width_profiles`.