Speed up `GSPDataSource.get_locations()`

[JackKelly]

If a constant number of GSPs are available across all timesteps then we don't need to go round a slow for loop. Instead we can just randomly pick GSPs in one go using something like np.random.choice(self.metadata["gsp_id"], size=len(t0_datetimes))

[peterdudfield]

Don't we need to make sure there are no nan's for that timeperiod. i.e for each t0 only pick GSP where there is data.
Perhaps ive missed something though ....

[JackKelly]

yeah, that's right, but we could do that as a one-off bit of analysis... i.e. manually check there are no NaNs in GSP data, then we can vastly speed up GSPDataSource.get_locations

[JackKelly]

I think this should come before #304 tbh, because this could be a simple win, with minimal code to modify