Improve logging (closes #71)

This improves logging in several ways. It adds the creation of several log files with varying levels of detail and different components (like Midas, Dask, Distributed) to ease debugging. To support this, the command line option --log-file is deprecated and replaced by --log-prefix, which gives only the prefix to be used to distinguish the output of one Midas run from that of another. In an HPC environment, this will usually include the job number.