Test case working again with new fluid.
Clean out dead uncommented code.
Remove more unused code.
Moved UpwindSelector out of anonymous namespace.
Added Selector class.

Used to choose element-by-element between two (potentially AD) vectors,
depending on signs of elements of a third vector. Simple and not optimized.
Can now solve with bhp-controlled wells.

Simple initial code. Assumes that well_state.bhp() contains
well bhp targets, does not check control structures.