Dclab

- enh: lift restrictions on valid options for [setup]:medium
(can now be any arbitrary string)

- feat: support the "image_bg" feature which contains the
rolling mean background image computed by Shape-In

- feat: new CLI command dclab-split to split a large dataset into
multiple smaller datasets

- fix: do not cache hierarchy child feature values; this might lead
to intransparent situations where a child has different features
than its parent (you cannot always rely on the user to call
`apply_filter`)
- fix: hierarchy child configuration section "calculation" was not
updated with the hierarchy parent values
- docs: add example for loading data from DCOR and computing
the Young's modulus

- fix: support unicode characters when writing HDF5 in Python2

- docs: add artwork
- fix: support unicode characters in sample names in Python2