Should refactor the Node (and other) enums to traits?

[jetuk]

The trait(s) would provide a nice interface and allow other crates to write their own components. This would be in the pywr-schema crate, and not pywr-core. It would remove a bunch of dispatching boiler plate at the expense of dynamic dispatch. The latter is probably not an issue for the schema which should not be a performance bottleneck.

The reason this was not before was because of the use of serde which can not (de)serialize trait objects. However, the the typetag crate can make this happen.