PCBase base class for python type preconditioners

[JHopeCollins]

Bumped up from https://github.com/firedrakeproject/firedrake/blob/main/firedrake/preconditioners/base.py, although this doesn't implement all of the methods in the Firedrake class because some of those need firedrake specific things (e.g. forms, or how we deal with the appctx).

The important two things are:

1. separating out the initialisation and updating methods and getting setUp to call initialise only on the first call. This just saves either duplicating work unnecessarily or having a really chunky if-block in setUp.
2. abstractmethod for the minimal interface of initialize, update, and apply.

[pbrubeck]

LGTM

[connorjward]

Otherwise very happy. This is great.

Changes implemented.