Prevent additional attributes from being added to system/moves

[rsdefever]

Is your feature request related to a problem? Please describe.
It is easy to make a typo, and instead of editing an attribute of the moves object, you actually add an extra attribute (and do not make the intended edit). Example:

moves = Moves()
moves.cbmc_n_dihed = 15
# This is what I might try to do...but with a typo,
moves.cmbc_n_dihed = 15
# No error or warning is thrown, so I think I've modified cbmc_n_dihed,
# but actually it still has the default value

Describe the solution you'd like
system and moves have well defined attributes. The users can in some cases edit those attributes, but I see no need to add additional attributes. It would be good to throw an error if the user tries to create a new attribute.

Describe alternatives you've considered
Open to ideas, none come to mind immediately.

Other information
In some ways this is related to #49; now that we are closing in on the desired UX for MoSDeF Cassandra, it may be time to think about creating a somewhat more robust backend.