Enable perses app to run solvent and complex legs at same time in different threads

[jchodera]

A very simple way to drastically improve user experience would be to enable the perses app use separate threads to run solvent and complex legs at the same time on different threads. It's possible that either the GPU or CPU would be useful for the solvent leg (especially if multiple CPU threads could be used). We would just need to ensure we use separate logging files for each, but we need to add an option to direct logging output to a file anyway.

[ijpulidos]

As discussed, it could be very useful and not too hard to have a way to specify the number of processes for each of the phases. As a subkey-value pair for the current phase YAML option. Doing this with multiple processes should be easier and would avoid hitting python's GIL.