Load simulations done with pfft/p3dfft without these libraries

It should of course be possible to load simulations done with pfft/p3dfft without these libraries.

Unfortunately there is no argument in load_sim_for_plot and load_state_phys_file to do that (as experienced by @jreneuve).

It seems to me that load_sim_for_plot and load_state_phys_file could set params.oper.type_fft to "defaut" by default.

However, I also think we should add an argument overwrite_params to these 2 functions to load simulations with modified parameters.

@jreneuve and @avmo, I would be happy to know your point of view.