Switch from dynamic dispatch to enum dispatch for python interface

[ljeub-pometry]

The idea here is to put all the different views we support into an enum which can be used as the base graph instead of the dynamic graph. This has several advantages:

• make it possible to implement reset and optimisations without having dynamic dispatch everywhere
• should make running algorithms from python faster as the graph is no longer dynamic

The main disadvantage is that adding new views from outside the crate is not possible as they have to be added to the enum.

[ljeub-pometry]

Our python interface still very much uses dynamic dispatch...