Allow restart functionality

[samhatfield]

Currently the model always starts from rest. Functionality to restart the model from a NetCDF file should be added.

[tenomoto]

I added a feature to initialize from a file. This is not strictly a restart. A true restart requires two time levels. The input format is the same as the output netcdf. If init.nc is found the state is initialized from the file.

[nbren12]

@tenomoto I've been working on something similar, but I'm getting some polar artifacts when initializing from interpolated era5 data and also when initializing from one of SPEEDY's own output files...I would be interested to know what solution you have.