feat: add pywr-schema-macro crate

[Batch21]

This contains a derive macro that implements parameters and parameters_mut methods for nodes.

[Batch21]

@jetuk I was able to copy over your code from the v1 schema with little modification. I've left in a TODO for parsing more complex node attribute types, such as factors and links.

[jetuk]

This looks good enough to unblock #99 .

It will need some more refinement to handle the other fields types, but that can be part of a wider improvement.