feat: Add VisitMetrics and VisitPaths traits and implementation.

This adds methods to the network and various components for
"visiting" Metrics and Paths. These methods are useful for finding,
for example, all paths in a model.

The traits are trivially implemented for common types. New derive
macros replace the previous PywrNode macro. These macros generate
implementations of the two traits for the struct and enum types
currently used in the schema.

Outstanding items:
- [ ] Should there be separate `visit_` and `walk_` methods. Where
walk would be recursive, but visit not?

pywr-schema-macros/Cargo.toml

@@ -18,5 +18,5 @@ proc-macro = true
 [dependencies]
 syn = "2.0.52"
 quote = "1.0.35"
-
-
+heck = "0.5.0"
+proc-macro2 = "1.0.81"