add optional n_processes kw to backtest_many

[quant5]

Solution to #177

Tested on my machine running Windows and Python 3.11.5.
Original behavior (which errors for me, hence #177) is retained if parameter is not supplied.

@enzbus It's up to you whether to keep multiprocess. (currently, it's installed as a dependency.)

I hope the formatting changes are ok - my formatter (black) automatically does those.

[enzbus]

Looks like there's a ton of changes because your editor ran black? I was thinking to set number or processes to something like min(cpu_count(), n) where n is the number of policies passed to backtest_many. Let me have a closer look though. Good that you found the fix to #177 anyways.

[quant5]

yeah, the only actual change is here https://github.com/cvxgrp/cvxportfolio/pull/179/files#diff-ceed8a0828d51fb92ef677933274ae79b2d6f6f8082281b8fb449b6aae636892R843

your approach of min() is fine by me as well.

[enzbus]

I just merged in master the code that uses min(cpu_count(), len(policies)), I think it's good. Not sure though, leaving this PR open since it may be useful to provide num_processes explicitely.