Add restart method on Case

[dafyddstephenson]

The method should return a new Case instance whose start_date corresponds to end_date on the previous case. It should also accept a new_end_date argument.

The only issue is that, in the case of spinning the model up, typically the user would want to change the timestep size, which is defined separately for each component (in our case this involves updating ROMSComponent.discretization.timestep). I think this is acceptable for now.

[dafyddstephenson]

The big problem here will be automatically replacing ROMSComponent.initial_conditions with the correct file path to the restart file. C-Star will somehow have to work out what the name of the restart file is going to be (presumably by parsing the AdditionalCode.source_mods / ocean_vars.optfile to get the restart file frequency and combining this with the time step, run length, and run name specified in the ROMS namelist file).

[dafyddstephenson]

Related: #114