Accommodate other SWMM objects and variables

[barneydobson]

These are hardcoded. That's ok if this is all there is and there won't be any more, but just checking that we don't want to make this changeable.

Originally posted by @dalonsoa in #58 (comment)

Currently we only support Subcatchments, Nodes (as storage type), Links (as circular type) object types.

We also only store information for flooding, depth, flow, runoff.

Future applications may want to expand on both of these fronts - any discussion of that can be kept here

Should be easy to accommodate now with: #84