Allow solver to be supplied by clients instead of hard-coding it from IDAES or WaterTAP

[lbianchi-lbl]

From the Apr 29 working session meeting notes (paraphrased):

• Solver should be passed as argument to (FIXME: which function/class?) by clients
• Possible "80/20" solution to provide good flexibility without too many headaches on our side: supporting HasSolveMethod | str | Callable[[], SolverFactory]
  • has .solve() method: use directly
  • str: call SolverFactory
  • callable: call it