INLA - Allow spatial dependency to be dataset specific

[Martin-Jung]

Currently the add_latent_spatial() for INLA only add a single spatial latent effect as SPDE to the model that is then shared in case multiple likelihoods are specified. This thus assumes that there across datasets/likelihoods there is a single source of spatial dependency/errors that are shared by all datasets.
Another option would be to have individual new latent effects that are shared but specific for each dataset. This could be added as a parameter add_latent_spatial() (maybe separate_spde = FALSE ?)
So
f(spatialfield1, model=spde, hyper=priors)
and
f(spatialfield2, copy='spatialfield1', fixed=FALSE, hyper=priors),

[Martin-Jung]

Partially a duplicate of #7