Fedor restart

Implemented restart functionality. See test_restart for an example. I can add a notebook if required, but there's not much too it. You can also specify a restart file in mdu to speed up initialisation, this is done using a cold start, the api uses a warm restart (overwrites waterlevels and currents). 
I also replaced the manual float, with the numpy testing (npt by convention). This somehow ended up in this branch.