Snap polygons updates

[tmchartrand]

Addresses #190 and #189
much of this is dedicated to fixing and improving the FromLimsSource functionality (#190) :

• updating the argschema references to the current naming
• updating the schema to use the AllenSDK-matched environment variable credentials as defaults, and fixing some other mismatches between schema and code
• expose the parser so it can used by other code not running snap_polygons as a script
• fix the LIMS query to not select duplicate layer drawings (this is the norm, not an exceptional case)

also addresses #189 by:

• providing an alternate code path that doesn't require pia/wm paths, skipping the trimming of layers to those paths' bounds
• clarifies the behavior of find_vertical_surfaces, which also uses the pia/wm paths when available to annotate the top of L1 and bottom of L6
• also updates a few other docstrings that had confusing or wrong descriptions of steps in the process.

[CLAassistant]

All committers have signed the CLA.

[tmchartrand]

Updating to say that the above test failure is in test_post_data_to_s3, not related to the changes here.