Use a water depth map from one model run as input for another

[dvalters]

See also #34 but beware this wouldn't give comparable results necessarily - see Tom's comment in that issue.

Some extra follow up points from email discussion:

• For the digital twin/forecasting we’re doing this should work fine as we’re starting from the best known representation of what’s happening in reality.
• For stopping/restarting the model I’m sure it is technically possible to do this as the Met office (and other institutes/climate models) periodically save their entire simulation state on long model runs in case they need to be interrupted.
• It may require another solution than just dumping everything to a text file – but I’m not sure what that solution is.
• You won’t get the same number of DP saved to the file as stored in the memory (double) for all the parameters… If you can – it might work, but suspect if just one wee thing is a bit different the overall outcome will be similar – but not the same. Deterministically non-linear model

Acceptance Criteria

• [x] Model now takes a water depths file AND a flow vectors file, which are used as initialisation DEMs
• [x] Model produces similar results when using the prestart water files compared with single run uninterrupted

[dvalters]

Update: Not sure it needs the vectors file, at least for a first pass implementation (the vectors will establish themselves after a few timesteps of loading the water depths, we aren't looking for 100% reproduction, just a quick way of loading spun up conditions into a catchment.)

[dvalters]

Merged in: #41