Nanite

- feat: allow to specify geometric correction factor k during fit
(tip position is multiplied by k and contact point is modified
directly before and after fit)

- feat: allow to load fitting models from external Python files
- enh: add method to deregister NaniteFitModels

- docs: added some clarifications in the model docs
- enh: check for leading/trailing spaces in models
- enh: add more checks during loading models

- feat: introduce new NaniteFitModel class
- ref: deprecated get_anc_parms in favor of compute_anc_parms
- docs: add more information on how to write own model functions

- enh: more sensible default parameters for POC estimation fits

- feat: allow specifying the minimizer method for fitting (21)
- feat: new contact point estimation with linear fit for baseline
and polynomial fit for indentation part
- feat: new contact point estimation with Fréchet distance between
curve and direct path in normalized coordinates
- tests: rename experimental test data files according to
afmformats scheme (20)