Dcnum

- fix: when checking for ppid kwargs, allow kwargs defined in `__init__`
- ref: use kwonly arguments for segmenter `__init__` method
- tests: set environment variables so that libraries use one thread only

- feat: implement "copy" background class
- fix: handle small kernel size before background computation (16)
- fix: update experiment:run identifier for dclab-user convenience
- fix: make sure output directory exists for pipeline job
- fix: forward debugging flag to SegmenterManagerThread
- ref: reduced minimum frame count for rolling median background
computation to kernel size

- BREAKING CHANGE: Remove preselection capabilities, because it is not
well integrated into the pipeline. For more information, please see
issue 15.
- feat: introduce logic submodule for running pipelines
- feat: implement HDF5Writer.store_log
- enh: add Segmenter.hardware_processor property
- enh: introduce pipeline identifier for data pixel size
- enh: reduce pixel_size accuracy to 8 digits after the decimal point
for pipeline reproducibility
- enh: warn the user when creating a basin-based file without basin paths
- ref: deprecate pixel size correction in HDF5Data
- ref: increment DCNUM_PPID_GENERATION to 7
- ref: several changes and deprecations in the PPID helper functions
- build: change flat-layout to src-layout (issues with editable installs)

- fix: protected features (e.g. time, frame) were not included in ouptut
files when the input file was basin-based
- ref: increment DCNUM_PPID_GENERATION to 6

- fix: correctly raise KeyError for missing image-based feature from
HDF5Data._image_cache

- fix: properly convert variable-length string logs in `copy_metadata`