forward_model: DistributedForwardModel created which uses the multiprocessing
               package for distributed computations.

Phase maps now also have a mask and a confidence matrix.
Began new script structure (temporary folders created).
Cleanup, logging and documentation done.
config: NTHREADS is now set here.
dataset: Method for creating 3D masks added.
phasemap: Mask and confidence matrix added. Padding and cropping supported.
forwardmodel: Stubs for multiprocessing added (next TODO!).
kernel: Added support for perturbed reference wave.
magdata: Padding and cropping supported.
projector: New property 'sparsity' introduced.
regularisator: Stub for ComboRegularisator added (next TODO!).
tests: Changed accordingly.