Dclab

- maintenance release

- enh: add identifier to AncillaryFeature class
- enh: introduce MachineLearningFeature to simplify the use of
machine learning models in ancillary features
- fix: load_modc should return a list of models
- ref: rename `rtdc_dataset.ancillary` submodule to `anc_feat_core`
- ref: rename `rtdc_dataset.plugins` submodule to `anc_feat_plugin`
- ref: move the `ml` submodule to `rtdc_dataset.anc_feat_ml`
- ref: move tensorflow-related code into its own submodule
- setup: bump minimum Python version to 3.7

- fix: ignore defective feature "volume" in pre 0.37.0 .rtdc files
- fix: always perform version branding when using RTDCWriter context
manager
- ref: silence numpy deprecation warning in RTDCWriter

- enh: speed-up contour computation by a factor of three
(volume is computed by factor of two faster)

- enh: make IntegrityChecker class available on top level module

- BREAKING CHANGE: The volume feature in all previous versions was
overestimated by about 2µm³. Please re-run your analysis pipeline.
- fix: volume was computed incorrectly (141)