move some functionality to sub-modules

[isaacsas]

Catalyst exports a lot of stuff currently, and some of it could be moved to sub-modules (or libraries) to clean up and reduce the default exports (network analysis functionality beyond conservation laws, spatial functionality, etc). Given our next release will be breaking this is perhaps the time to make such a switch.

[TorkelE]

If we do split the library I'd rather not rush it. I think it would be better to simply do the current breaking release asap, and then look at what a split could be later on without stress.