Add "sub logs"

[adamltyson]

Is your feature request related to a problem? Please describe.
The aim of fancylog is to be able to log everything that may be required to debug software when it goes wrong, and reproduce it when it goes right. If we log everything into a single file it could get very hard to parse quickly. It would be useful to add the concept of "sub logs" for specific computations. I.e. the end result could be something like

• Main log
  • Process 1 log
  • Process 2 log

Describe the solution you'd like
There should be a way of easily creating new logs that are linked to the main log (and then going back to the main log)

Additional context
It would also be useful to be able to direct the logging, stdout and/or stderr of third party tools to a specific log. E.g. brainreg has specific NiftyReg logs that would be better included in some way within fancylog logging.