Callback to save the iterate every n iterations

[MargaretDuff]

Description

This would be a useful callback for some of our users.

• It should have options to take in n=how often the iterates are saved
• It could also have options to take in a CIL writer to save images or by default store in a dictionary or numpy array? A dictionary would allow the iteration number to be saved too?

Environment

import cil, sys
print(cil.version.version, cil.version.commit_hash, sys.version, sys.platform)