Grid collection

[chrishavlin]

This PR:

• refactors the block collection to create an abstract base class for collections
• adds a GridCollection object for directly rendering a list of covering or arbitrary grid objects

Close #217

To do:

• add some figures to this PR
• update docs
• get the block outlines working for GridCollection

example outputs

Here are the outputs of the test with a list of arbitrary covering grid.

for cartesian coords:

for spherical coords:

And the output of examples/grid_collections_enzo_64.py, which loads 2 covering grids:

[chrishavlin]

the cartesian covering grid test image is empty

[chrishavlin]

grid outlines working -- also removed the block outlines menu item for this rendering object since they are not applicable.

[chrishavlin]

oh, hit undraft too soon maybe. forgot i need to double check the plain covering_grid behavior

[chrishavlin]

well, very confused about why the cartesian covering grid test case saves an empty image... running the same code locally with osmesa returns the expected figure

it's only when running it in pytest that it is empty.