Dclab

- enh: implement function for obtaining the description of a
configuration key get_config_value_descr

- enh: implement dumping of Configuration to json

- enh: allow to compress, condense, and repack without checking for
the input file suffix
- docs: fixed example data (sphinx encountered unexpected warnings)

- fix: ignore empty logs (raised IndexError before)
- tests: increase coverage (136)

- enh: support new configuration keys for online polygon filtering
in the form of "[online_filter]: area_um,deform polygon points" (134)
- enh: new helper functions in dclab.definitions: get_config_value_type,
get_config_value_func, and config_key_exists
- ref: issue a warning when the data type of a configuration key is
not correct

- fix: make get_job_info and get_command_log available in cli submodule
- tests: add documentation for test data (119)