Nanite

- ref: renamed the mode `model_hertz_parabolic` to
`model_hertz_paraboloidal` to be consistent
- docs: updat code reference and other minor improvements

- fix: `preprocessing` keyword not working in `Indentation.fit_model`
- docs: add another scripting example and minor improvements
- tests: increase coverage

- ref: remove legacy "discrete" feature type
- ref: renamed kwargs for `Indetation.rate_quality`
- ref: new method `nanite.load_group` for loading experimental data
- ref: new class read.data.IndentationData for managing data
- ref: replace dataset.IndentationDataSet with group.IndentationGroup
to avoid ambiguities
- fix: add missing "zef18" training set
- fix: sample weight computation failed when a rating level was missing
- enh: add `nanite-generate-training-set` command line program
- tests: reduce warnings and increase coverage
- cleanup: old docs in nanite.rate.io
- docs: major update using helper extensions

- initial release