Allow offsets and latent effects to be dataset type specific

[Martin-Jung]

Currently both the add_range_offset() function and the add_latent_spatial() function add the offset, respectively the shared SPDE to all equations regardless of dataset type.
Ideally there is a dataset= ... and group= parameter or similar to allow having different offsets per dataset and furthermore to have the option to have shared or independent spatial latent effects.

[Martin-Jung]

Latent effects partly addressed in 53002f2 . Offsets still to be corrected

[Martin-Jung]

With commit 56b69ea it is now possible to specify and add multiple offset to any model object.
And transformation of the offsets has to be done before adding them to the model (for instance via log(offset)). Multiple offsets are summed into one.