Move grid_search code to separate repo

[JamieTaylor-TUOS]

Move the code in pvoutput/grid_search.py to a separate repo.

Context

Code in pvoutput/grid_search.py is sufficiently generic to warrant moving to a separate project and making pip installable.
This would simplify the pvoutput repo somewhat.

Possible Implementation

Move to separate repo (either SheffieldSolar or openclimatefix) and make pip installable.
Add new lib to requirements.txt and setup.py

[JamieTaylor-TUOS]

See also #79

[peterdudfield]

It certain is useful to have it within this repo.
But I can also understand that this code, might be useful independently of 'PVoutput' - probably.

Im happy to make a repo and move it over there, what do you think @JamieTaylor-TUOS ?