Continue previously unfinished runs

[BramDr]

When running long high-resolution PCR-GLOBWB simulations, it is often the case the the simulation time exceeds the time allowed by the slurm manager on external servers. Or, it may be that there is some other error that causes (longer) simulations to stop. It would be nice if we could continue previously unfinished runs, using the same output directory.

Currently, PCR-GLOBWB appends its outputs to the output NetCDFs per timestep and stores its states to the state pcraster maps per year. An argument should be added to the PCR-GLOBWB runner (e.g. --continue), where the model does not clean up the output directory. Rather it checks:

1. If the output files already exist
2. If the state files already exist
3. The latest available state file

And than continues the previous run from the latest available state file, appending to the output files when necessary.

[BramDr]

Added by pull request #41