Implement input/output member functions in simulation

[mppotter]

Implement: save, load, plot_field, progress_bar into simulation class

save: write all fields to disk at a checkpoint timestep
load: load fields into sim from a particular timestep
plot: generate an image from the fields variable
progress: estimate total time remaining/elapsed for sim

[mppotter]

Expanding this ticket to also include input checking for:
set_dimensions
add_field