save_fn mandatory for pocomc?

[pmcdonal]

This seems to make save_fn mandatory for pocomc,

desilike/desilike/samplers/pocomc.py

Line 80 in 453a853

if self.save_fn is None:

in which case it would be good if the help said (and ideally function signature, i.e., can't you put ,*, in argument list and then after that a word without default will be mandatory keyword? ... or something like that)