New parallel option that is fully consistent with FlorisModel

[misi9170]

This pull requests provides a new interface for parallel computing with FLORIS. The existing ParallelFlorisModel was not fully consistent with the new FlorisModel and UncertainFlorisModel developed for FLORIS v4. This addresses the inconsistency in a new class, ParFlorisModel. ParallelFlorisModel will remain in FLORIS v4 to avoid making breaking changes, but now has a deprecation warning and will be removed in the next major version release of FLORIS.

This addresses #971 and possibly partially address #944.

Development still to take place:

• [x] Create script for timing both the existing ParallelFlorisModel and the new class with increasing n_findex and increasing n_turbines, and can be run on both HPC and locally. Report results.
• [x] Test whether there is a significant speed increase in returning powers only, rather than full velocity fields. Even if not, we may leave that option. [Minimal effect when using multiprocessing]
• [x] Test other parallel computing paradigms and see whether they'd be easy to include (concurrent, mpi4py)
• [x] See whether it is possible to spin up the worker pool on instantiation of the class and then send it jobs, to reduce overhead if multiple run() calls are needed [Yes, with pathos package]

Checklist before converting this draft to full PR status and preparing for merger:

• [x] Remove temporary examples designed to be run on HPC
• [x] Add documentation
• [x] Select name for new class and rename
• [x] Add deprecation warning to existing ParallelFlorisModel
• [x] Add any new examples
• [x] Replace usage of existing ParallelFlorisModel in examples (if it’s there)

[misi9170]

Something to decide on is the default parallel interface. My thinking is that the default should be "multiprocessing", even though "pathos" is faster, because the multiprocessing package is pretty ubiquitous and pathos is still on version 0.x

[paulf81]

Something to decide on is the default parallel interface. My thinking is that the default should be "multiprocessing", even though "pathos" is faster, because the multiprocessing package is pretty ubiquitous and pathos is still on version 0.x

I agree, using multiprocessing seems the better choice for default

[misi9170]

We have decided to leave out mpi4py support for now. If requested, we may consider including this in future.