Decouple process class from Model

[benbovy]

This is a follow-up on #59 towards a less invasive framework.

@process is now a thin wrapper around attr.s. Applied on a class A, it returns the class (almost) just as it was decorated with attr.s. The more invasive modifications (custom properties, etc.) are now in a subclass of A that is created programmatically and accessible from A.__xsimlab_cls__ (Model objects use the latter subclass instead of A directly).

The great advantage is that it is now possible to use instances of process-decorated classes independently of any Model, which is IMO a better solution than the approach proposed in #50 for testing the logic implemented in those classes.

Variables declared with intent='in' or intent='inout' are now included in__init__ generated by attr.

An alternative approach would be to stick with one single class and customize its __init__ to circumvent the limitation of read-only properties created for input variables (see, e.g., python-attrs/attrs#393 (comment)). This is maybe less complicated than the approach used here, but it still alters the class significantly.

TODO:

• check if this approach works with Include parent classes in process name/class reverse lookup #45 (subclass).
• set init=True and repr=True for all input variables
• add tests
• update docs (new "testing" section)
• update release notes