Changes in dynamical core

[fmaussion]

This PR is an attempt to increase the numerical stability of the numerical scheme.

Implemented so far:

• the mass-balance can now be computed only once a year or once a month on user request. I.e there will be no elevation feedback within a year any more. This makes the run faster (at least 10% faster), and also a bit more stable I hope
• plots now have a scale bar as legend (needs the very last salem)
• some IO changes (relatively minor, larger changes in a subsequent PR)
• give the user some control on the time-stepping scheme