Logging

Sometimes it is hard to know what FESTIM is doing and when it's failing.

We have _some_ print statements here and there but a lot is lost in between.

We would need a more robust way of logging what happens, with different logging levels (just like fenics has a log system).

I believe fenics uses [this](https://docs.python.org/3/howto/logging.html).

I think we can do something similar.