Standardize file type handling in gsd_utils

[chrisjonesBSU]

This PR does the following:

• Allows all of the functions in gsd_utils.py to handle either a gsd file and frame number, or work directly with a gsd.hoomd.snapshot

I added a _validate_inputs() function which both checks for what is being passed in (file or snap) and does some quick validation. The function returns a snapshot regardless of what's passed in. It gets called at the beginning of each of the util functions, which then work with snapshots.

• Re-names a couple functions

frame_get_type_positions renamed to get_type_positions so that it's shorter.
snap_molecule_indices renamed to snap_molecule_cluster so that it more closely matches the naming of freud.cluster.Cluster() which this function is basically a quick wrapper for.

• Adds a new function that returns all of the particle types in a gsd file/snapshot
  Nothing fancy, just a quick helper/utility function.

[codecov]

Codecov Report

Merging #6 (11a625b) into master (d7535cf) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master        #6   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         2           
  Lines           10        38   +28     
=========================================
+ Hits            10        38   +28     

Impacted Files	Coverage Δ
cmeutils/gsd_utils.py	100.00% <100.00%> (ø)

[jennyfothergill]

This is nice! Let's add tests for get_all_types and snap_molecule_cluster and maybe a test that _validate_inputs raises the error when all three args are provided.