2d likelihood scans

[alexander-held]

fit.scan currently only supports 1d likelihood scans. Add functionality for 2d scans (which could be set up as multiple 1d scans) and associated visualization. In that context it would also be useful to simplify fit parallelization for different grid points.

[alexander-held]

A script for simple 2d scans can be found at in this Gist, it works with the example workspace built with cabinetry (via example.py).