Move DART-specific constraints into a subdirectory about aikido

[psigen]

Does it make sense to invert this grouping? I.e. move these into:
aikido/dart/statespace
aikido/dart/constraint

This would make the DART-specific code all be in one namespace which could be a component (which many other components would probably depend on). I am not sure if this is useful or not, but it may make some of the use-cases @cdellin was proposing easier by decoupling DART as a hard dependency.

Specifically, now that statespace is its own thing, it's more likely that planners and trajectory representations are be decoupled from interfacing directly with DART.

[brianhou]

I'd like to do this since we've been working on cleaning up lately. I'm not sure whether it makes sense to invert this grouping, though. How useful do we think AIKIDO is without DART?

[jslee02]

I would prefer not to have a subdirectory for DART-specific objects because it's AIKIDO's mandatory dependencies. The general rule of AIKIDO I believe is to have a subdirectory for an optional component that depends on optional dependencies.