Add restart functionality

If the value of the parameter `startStep` is different from zero in the file `simParams.yaml` that is located in the simulation case directory, the fluxes are read from file at initial step.

If the simulation is started from a time-step different from zero, info about iterations and forces are appended to the existing corresponding files.

The Boolean flag `restart` has been removed from all the `simParams.yaml` files.

When running some test with on the cylinder at Re=40, it notice some things:
- as expected, the number of iterations for the Poisson solver is bigger when the simulation is restarted, than the number when running in one strike,
- the drag coefficient is different just after restarting (higher value), and remains different for some iterations.

The pull request should not be merged before @anushkrish agrees on the method and the implementation in cuIBM.