Import matplotcheck classes via root rather than modules

[lwasser]

This is not an ideal import convention pt.

Nathan - let's look into being able to import ANY of the mpc tester objects using
mpc.RasterTester, mpc.VectorTester, mpc.PlotTester

i think we can do this by modifying the init file or similar to what they do in geopandas??

Originally posted by @lwasser in #228

[lwasser]

this is an example of implementing this --
https://github.com/jhamrick/plotchecker/blob/master/plotchecker/__init__.py

[lwasser]

hey @nkorinek this would be a good easy thing to add to MPC. it will the also clean up our imports.
we just need to update the INIT file. can you work on this when you are back in the office?