support fitting creep-compliance data

[paulmueller]

nanite should be able to fit the approach and retract segments of creep-compliance data.

• This requires some refactoring and abstraction. How do we deal with the afmformats.AFMData class?
• Also related to Make force-distance segments integer-valued (unit8 or less) afmformats#15.
• This should also work for stress-relaxation and others (they are all segment-based, so it should be trivial)