Change of _maps attribute to non-hidden type.

1. Added Attributes section in Mapcube docstring.
2. Changed _maps to maps.
Fixed mapcube._maps references.

1. In sunpy/instr/tests/test_iris.py.
2. In sunpy/visualization/mapcubeanimator.py.
Fixed test_iris.py again
Tried to fix docstring indentation problem.
Merge pull request #880 from VaticanCameos/MapIssue853

Change of _maps attribute to non-hidden type. Issue #853