Include Tikhonov-prior versions of EKP.

[odunbar]

Issue

"Tikhonov" EKI in the literature, is achieved by applying the standard algorithms to an augmented state.
Namely, for tikhonov parameter a, and a prior N(m,C) the typical update is computed with the change:

• G(u) -> [G(u),u]'
• y -> [y,m]'
• Gamma -> [Gamma 0; 0 a*C]
  Currently we leave such changes to be completed by the user. However we could produce an interface for this, if it is of interest.

Possible Solution

• In one interface, the user provides additionally a Tikhonov constant a (default 1) and the prior distribution
• This must take place at/before construction of EKP
• The user will call update_ensemble!(...) with G(u) as usual
• During update_ensemble! the augmentations are applied to states.

When performing this internally (vs leaving it to user) imlementations must take care of additional locations where there is misalignments of state.

• compute_error! will need to include the prior term
• adaptive scheduler computations may require the prior term
• and possible index issues for update-groups

Finally one must note that

• UKI, currently has an impose_prior flag, and EKS, GNKI hav prior contributions by definition. So one may need to reconcile consistency here (e.g., impose_prior = true is the same as the Tikhonov with a=1?)