Add in conversions to useful types for some custom fields

[JamieMair]

Considerations

• Would these conversions allocate? If so we should think about adding a buffer, or a mitigation to handle this.
• Can these be done in the non-autogenerated code with some hand written functions?
• Keep in mind that MuJoCo uses CSR instead of Julia's CSC.

TODO

• data.qM - the mass matrix
• data.efc_J