Allow `FileReduction` to save DynamicsVariables types

[Alexsp32]

The underlying ComponentVectors can be stored in their flattened representation, which the FileReduction would convert into a (N_atoms * N_dofs * 2 components + electronics) x N_snapshots Matrix. We should then also offer a function to rebuild DynamicsVariables vectors which takes a user-provided "recipe" for DynamicsVariables (i.e. the same way they set up a simulation) to rebuild from this Matrix.