Split `DynamicsGraph` into separate files

This issue was imported from [github.gatech.edu](https://github.gatech.edu/borglab/GTDynamics/issues/7).
The original issue was opened by @aescontrela3 on 2020-02-18T06:02:54Z and assigned to @yzhang3333 @aescontrela3 @smccormick32
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Currently, `DynamicsGraph` is bloated and complex, containing code for trajectory optimization, graph visualization, single timestep dynamics graphs, etc. To decrease code complexity this class should be split up into multiple classes and perhaps some utility files.

I agree. @yzhang3333 have discussed splitting DynamicsGraph into multiple separate files (e.g. trajectory stuff, dynamics graph stuff, etc.), but those changes are out of the scope of this PR. I'll make an issue regarding this.

_Originally posted by @aescontrela3 in https://github.gatech.edu/borglab/GTDynamics/pull/6/review_comment/create_