Dclab

- maintenance build

- setup: bump imageio to 2.8.0 for Python>=3.4
- ref: removed NoImageWarning during export (warning is already
issued by fmt_tdms.event_image)

- feat: introduce new feature names `ml_score_???` where `?`
can be a digit or a lower-case letter of the alphabet (77)
- feat: introduce new functions `dclab.definitions.feature_exists`
and `dclab.definitions.scalar_feature_exists` for checking the
existence of features (including the `ml_score_???` features
which are not in `dclab.definitions.feature_names`)
- feat: introduce ancillary feature `ml_class` which is
defined by the `ml_score_???` features
- enh: fmt_dict automatically converts scalar features to arrays
- ref: replace check for `dclab.definitions.feature_names`
by `dclab.definitions.feature_exists` where applicable
- ref: replace access of `dclab.definitions.feature_name2label`
by `dclab.definitions.get_feature_label` where applicable
- ref: do not automatically fill up all the box filtering ranges
in `RTDCBase.config["filtering"]` with zeros; raise ValueError
if user forgets to set both ranges
- docs: major revision (promote Shape-Out 2 and DCOR)

- fix: `kde_methods.bin_num_doane` now uses 5 as default if it
ecounters nan or zero-division
- docs: updates related to Young's modulus computation

- enh: cache more online data in fmt_dcor
- enh: add `dclab.warn.PipelineWarning` which is used as a parent
class for warnings that a user might be interested in
- fix: temperature warnings during emodulus computation revealed
only the lower temperature limit of the data

- feat: implement volume-deformation isoelasticity lines (70)
- fix: specifying an external LUT as ndarray did not work
- scripts: finish 'fem2iso_volume.py' for extracting volume-
deformation isoelasticity lines
- scripts: add 'pixelation_correction.py' for visualizing
pixelation effects on area_um, volume, and emodulus
- ref: renamed isoelasticity line text files