Move FastScape restarting functionality into the ASPECT checkpointing framework

[anne-glerum]

At the moment, the FastScape interface does it's own checkpointing and restarting. Its current implementation means that:

1. The FastScape and ASPECT restart files could be out of sync if a crash occurs after calling FastScape and before writing an ASPECT checkpoint.
2. fastscape.cc doesn't read in the restart files during its initialization, but during the first call to compute_velocity_constraints_on_boundary(). However, other plugins might want to query the interface for the value of checkpointed variables before that. For example, I'm setting compositional field boundary conditions based on FastScape info, and these constraints are computed before mesh deformation.
3. Checkpointing FastScape interface variables only works if 'Steps between checkpoint' are used instead of 'Time between checkpoint'.