Share any object between process classes

[benbovy]

There are cases where we need to share objects between processes that are not "variables" (e.g., callables, instances of custom classes such as particles or agents in Lagrangian or agent-based modelling). It is already possible to declare such objects using xs.variable, but that doesn't represent well what is shared in those cases, i.e., the dimensions may not be relevant.

It might be good to provide another attr.attrib wrapper for those objects, e.g., xsimlab.object. For the xarray extension, it would be also clear that those objects can't be directly used as inputs / outputs (they first need to be converted somehow from / into a variable).