Add weir

[SouthEndMusic]

Fixes #347.

See #482 what has to be done related to this.

[Huite]

While I'm no longer intimately familiar with the code base, this looks good to me -- the change is also somewhat minor as the weir is arguably just a Pump alias.

I think "weir" is maybe not the best term: it's explicitly a weir that maintains the level. I don't think the name has to be a blocker, but maybe it's worth keeping an issue for it.

What would be good to change before merging is to resolve the auto-generated code coverage remarks. Ideally the output is tested as well, but this is often difficult (hence 100% coverage doesn't mean 100% reliable). However, given that Julia is a dynamic language, it would be good that (nearly) all lines run during a test.

[SouthEndMusic]

Many of the changes in this PR also affect Pump (mainly in jac.jl). Not all of it is tested, but this is covered in #482.