Add parallel save/restart functionality

[CINTROINI]

This issue aims at adding the possibility to save simulation variables at specified iterations in order to restart calculations later from the same iteration.

The feature should support parallel execution and allow restoring the full simulation state consistently across processes.

Constraints:

• Same spatial discretization between restart calculations
• Same number of CPUs (MPI ranks)
• Same variable names

[CINTROINI]

How to Perform a Save/Restart Calculation

1. Saving Variables

The iterations_list_save_gf parameter (type std::vector<int>) of the PostProcessing object allows users to list the iterations where variables are saved as MFEM GridFunctions in a file under a format designed by MFEM.

All files are stored in a folder named "GF" by default, but this can be customized by the user with the gf_folder_path parameter (type std::string) as follows:

std::string gf_folder_path = "Restart";
Parameter("gf_folder_path", gf_folder_path)

All files are named in the following format:
'variable_name'_'iteration'.gf.'CPUrank'

For example, here is the file for the variable eta at iteration 2 on the 11th CPU:

eta_2.gf.000010

This approach implies the following constraints for the reading step:

• The spatial discretization must be the same between restart calculations.
• The number of CPUs (MPI ranks) must be the same.
• The variable names must be identical.

2. Reading Variables

Restarting a calculation is straightforward, but users must ensure consistency between the time and iteration at which the calculation is resumed.

The initial_iteration parameter (type int) must be added to the Time object:

Parameters(Parameter("initial_time", t_initial),
           Parameter("final_time", t_final),
           Parameter("time_step", dt),
           Parameter("initial_iteration", 2));

The initial conditions of the variables must be defined with reference to the saved file:

std::string file_restart = "eta_" + std::to_string(2) + ".gf";
ac_eta = VAR(&spatial, ac_bcs, "eta", Glossary::PhaseField, 2, std::make_tuple(file_restart, gf_folder_path));

[CINTROINI]

Add test covering Save/restart features

Test based on Ostwald ripening test described on PFHub website.

Ostwald test not described yet in the documentation because some numerical methods are currently being tested on this test.

This test is configured to perform the base simulation and the restart simulation with the same input data file