Add support for radial velocity data

[sefffal]

We would like to be able to support radial velocity data with the following requirements:

• Multiple instruments
• Instrument specific jitter
• Plotting support in the analysis file (timeplot and timeplotgrid)

We can implement this by creating a new subtype of AbstractObs (types.jl) and likelihood functions (models.jl)
Users can add an rv parameter to the system to model the unperturbed radial velocity.

We would like to be able to add rv data to both the system as a whole and to planets, so we will have to write two separate likelihood functions (they can share code though).

A nice bonus feature could be a helper function to read in the GAIA RV from the HGCA catalog that is already downloaded.

[sefffal]

Support for single instrument RV data now implemented.
Still todo: multiple instruments, docs, and recommendations for parameterization e.g. i=0, \Omega=0.

[sefffal]

I think we can say this is done now, and we can punt on multiple instruments for the time being.