Refactor process execution during a simulation

[benbovy]

Closes #40.

Also provides more flexibility for #52.

This refactoring is also a step towards making the framework less invasive. Ideally, a process class could be also instantiated and used independently of any model. This is important for testing purposes! Currently it is supported for "runtime" methods but we need a solution for __init__ too.

TODO:

• set runtime context dictionary and use it in xarray driver
• remove Model.initialize(), Model.run_step() etc (no depreciation)
• add tests where it is needed
• update docs
• update what's new