Dclab

- 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__`)

- maintenance release

- maintenance release

- maintenance release