- 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`