Dcnum

- fix: if the writer dequeue fills up, stall the feature extractor
- enh: optimize DequeWriterThread loop
- enh: minor optimization in HDF5Writer.require_feature

- fix: correctly handle mask images with no background on border
- fix: enforce user-defined features in concatenated_hdf5_data
- fix: Gate.features returned duplicate entries
- ref: cache empty border image creation for clear_border
- ref: rename internal background worker classes
- ref: simplify Gate initialization
- ref: Gate instance does not have to keep a reference to the data
- ref: better box_gates management for Gate class
- tests: increase coverage

- fix: replace unreliable Queue.empty and Queue.qsize (macOS support)

- fix: there was no progress for background copying
- enh: detect data files that contain no events
- tests: add test for sparsemed bg worker
- tests: add test for logic json encoder

- enh: define valid DCNumJobRunner state
- enh: more robust computation of progress
- enh: use HDF5Data when loading input data for background computation
- enh: automatically split segmenters and extractors equally
- ref: reduce default image cache size from 5 to 2
- ref: move dataset generation default kwargs to writer submodule
- ref: warn above 0.5% of discarded events in EventExtractorManagerThread

- fix: ignore empty HDF5 datasets when copying metadata
- fix: logging from subprocesses did not work as expected
- enh: warn user about total number of invalid masks
- enh: introduce DCNumJobRunner.error_tb for errors happening in threads
- enh: improve logging verbosity
- enh: append job information as log entry in DCNumJobRunner output file
- enh: set chunk size for all feature data to 1MiB in HDF5Writer
- ref: removed close_queues argument from EventExtractor init
- ref: rename event_count with image_count in background computation
- ref: do not print anything to stdout when computing background data
- ref: use data from background computer in DCNumJobRunner.get_status