Start richer logging

[ageorgou]

Addresses #33.

So far:

• Adds a description of how the new feature will work, to be included as a tutorial
• Refactors the existing logging to:
  • allow multiple sub-loggers (as we did in tests and demographics)
  • keep the functionality of "writing" of the population variables of interest as a separate class

Still needed:

• actually parsing the configuration and determining what should be written at each step!
• writing of the population at each step
• moving the tutorial to the docs directory (dependent on Improve Sphinx documentation #91) and adding it in the table of contents so it's part of the generated documentation

[pineapple-cat]

Tutorial looks good and examples are clear, I'll finish up with moving the updated Sphinx docs and then when everything is merged I'll put the tutorial in the right directory.