Support joint estimate over multiple locations

[seabbs]

Joint fitting across multiple locations may make sense when trying to pool information and is a potentially interesting extension (as it allows the exploration of sequence vs spatial information vs combined).

To implement this feature would need changes to the package code to support a location column in all preprocessing and postprocessing functions. Would also require a fairly extensive nontrivial rewrite of the modelling code that would be both fairly technical and require careful thought about the type of spatial relationship to include.

Note: Not part of a current research project.