Standardize UKI constructor to be consistent with EKS, document kwargs

[ilopezgp]

The UKI constructor requires α_reg and update_freq as arguments that do not have a default value. It would be good to make them keyword arguments with default to make it easier for users to use an off-the-shelf version, with a similar constructor to EKS (so, just an initial mean and variance).

I think α_reg=1 by default, but what about update_freq?

In addition κ and β are not defined, and there is a TODO flag within the constructor. It would be good to clean this up.

[ilopezgp]

Closed by #86 .