Logging

Add proper support for logging levels compatible to the `logging` module from the Python standard library.
* add command line switches `-l`/`--log-level` to `utils.parser`
* add a `logger` module controlling the PyOP2 logger
* add functions `debug`, `info`, `warning`, `error`, `critical` to the `op2` namespace
* factor out `MPIConfig` to its own `mpi` module
* replace `print` calls in the pyop2 library