Dclab

- 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

- fix: appending data to an hdf5 file results in a broken "index"
feature (re-enumeration from 0), if the given dataset contains
the "index_online" feature
- enh: allow to set external LUT files or LUT data when computing
the Young's modulus with the `lut_data` keyword argument in
`dclab.features.emodulus.get_emodulus`.
- ref: refactored `features.emodulus`: New submodules `pxcorr` and
`scale_linear`; `convert` is deprecated in favor of `scale_feature`

- setup: include Python 3.8 builds and remove Python<=3.5 builds
- scripts: renamed 'extract_lut_and_iso.py' to 'fem2lutiso_std.py'

- fix: `ConfigurationDict.update` did not take into account
invalid keys (everything is now done with (__setitem__`)