Dcnum

- enh: elevate `HDF5Data`s `index_mapping` to pipeline identifier status
(this changes the pipeline identifier)
- enh: improve sanity checks for `BackgroundRollMed`

- BREAKING CHANGE: mask postprocessing did a morphological opening instead
of a morphological closing, failing to remove spurious noise
- BREAKING CHANGE: perform first fill_holes and then closing_disk in mask
postprocessing
- feat: allow to specify ranges when creating an HDF5Data instance to
enable e.g. processing only a portion of an input file
- feat: volume computation via contour revolve algorithm (23)
- feat: background offset (flickering) correction via the
"offset_correction" keyword for the "sparsemed" background computer
and "bg_off" everywhere else
- enh: allow creating HDF5Writer from h5py.File
- fix: mask postprocessing did a morphological opening instead
of a morphological closing, failing to remove spurious noise
- fix: remove mask ppid part for segmenters that do not use it
- fix: mask postprocessing with "fill_holes" using `cv2.floodFill`
sometimes segmented the entire frame if the upper left pixel was not
set to background
- enh: perform first fill_holes and then closing_disk in mask
postprocessing
- enh: pop read-cache items before adding a new one
- enh: allow to request the raw contour from `moments_based_features`
- ref: increment DCNUM_PPID_GENERATION to 8
- ref: added new super class `BaseImageChunkCache`
- ref: use HDF5Writer in Background class
- ref: minor cleanup
- ref: rename submodule `feat_moments` to `feat_contour`
- ref: remove unused `name` property from `Background` class

- fix: make sure unsupported features are not propagated in
`concatenated_hdf5_data` (27)

- ref: remove "bg_med" and "index_online" from protected scalar features,
because "bg_med" may change due to a different background computation
method and "index_online" enumerates events from online segmentation

- feat: allow to register topical search paths
- ref: remove deprecated `get_ppid_from_kwargs` methods
- ref: remove deprecated `preselect` and `ptp_median` keyword
arguments from `QueueEventExtractor`
- ref: remove deprecated "key" from `get_class_method_info` info dict
- ref: issue UserWarning instead of DeprecationWarning when checking
segmenter keyword arguments
- ref: remove pixel size check for HDF5 data
- ref: remove unused `_get_model_file` from GPUSegmenter

- fix: correctly set number of workers for CPUSegmenter
- enh: update list of environment variables that should be set to
disable multithreading in subprocesses