Dclab

- ref: using temperature values outside the range for viscosity
computation now issues a warning instead of raising an error;
warnings were added for the CellCarrier buffers
- fix: handle number detection correctly in get_emodulus

- fix: always return an array when computing the KDE
- ref: make accessible static function RTDCBase.get_kde_spacing

- feat: compute elastic modulus from "temp" feature (51)
- enh: computing isoelastics from datasets can use
[setup]: "temperature" to compute the viscosity/emodulus (51)
- enh: define new meta data key [setup]: "temperature"
- docs: add an advanced section on Young's modulus computation (51)

- fix: hierarchy children did not pass `force` argument to
hierarchy parent when `apply_filter` is called
- fix: revert histogram2d "density" argument to "normed" to support
numpy 1.10 (Shape-Out 1)
- fix: implement unambiguous `RTDCBase.__repr__`

- feat: added better contour spacing computation based on percentiles
(dclab.kde_methods.bin_width_percentile)
- feat: add feature "index_online" which may be missing values (71)
- feat: implement __getstate__ and __setstate__ for polygon filters
- fix: write UTF-8 BOM when exporting to .tsv
- enh: add check whether unique_id exists in PolygonFilter

- fix: correctly handle filtering when features are removed from a
dataset
- ref: move dclab.rtdc_dataset.util to dclab.util
- ref: minor cleanup in computation of viscosity (support lower-case
`medum` values, add `dclab.features.emodulus_viscosity.KNOWN_MEDIA`)
- ref: cleanup dclab.rtdc_dataset.filter (use logical operators,
correctly display nan-warning messages, keep track of polygon
filters, add consistency checks, improve readability)