Use multiple processes per DataSource in Manager

[JackKelly]

Some DataSources would benefit a lot from having multiple Processes per DataSource.

Maybe make the n_processes configurable.

[JackKelly]

I'll work on this now...

[JackKelly]

Hehe... making leonardo sweat...

[JackKelly]

with 4 processes per data source, we're getting 18 batches of satellite data in 131 seconds. Which is 7 secs per batch.

[JackKelly]

with 8 processes, it's quite juddery and does about one sat batch every 9 secs.

[JackKelly]

with 1 process it's also 8 seconds per batch!