Dclab

- fix: spelling of Young's modulus (159)
- ref: define minimal R version 3.6.0 in lme4 submodule and
propagate any errors with possible solutions to the user
- ref: extract exact version string for R

- enh: cache computed file hashes with additional os.stat info
- tests: fix tests on Windows

- fix: handle empty trace feature in RTDCWriter.rectify_metadata

- fix: do not close H5File object passed to RTDCWriter.__init__

- fix: write FeatureSetNotIdenticalJoinWarnings to logs in dclab-join

- fix: KeyError object 'index_online' doesn't exist when joining
two datasets with dclab-join (158)
- fix: generalize online_filter configuration keys (156)
- fix: relax validation of online_filter configuration key types
- fix: make sure that only common features of the input files for
dclab-join are written to the output file (157)
- enh: add `with_unit` keyword to `get_feature_label`
- ref: slightly modified online_filter configuration descriptions