[graph-tool] Save state to disk for long running nested block state analysis

G'day! Just wondering whether it would be possible to implement, e.g. as a parameter of the minimize_nested_block_model function, a way to save intermediate results to disk at certain intervals and to resume the analysis, e.g. after a forced shut-down, or any other interruption. If I'd have some hints I might try to have a go at this feature myself. Or is this already possible with some lower level functions? Cheers! Felix --