Dclab

- enh: ignore defective feature "aspect" from Shape-In 2.0.6 and 2.0.7
- enh: support loading HDF5 data using the 'with' statement
(e.g. `with dclab.new_dataset(rtdc_path) as ds:`)

- fix: add numpy build dependency (setup_requires)

- fix: HDF5-export did not re-enumerate "index" feature

- fix: support nan-valued events when computing quantile levels
in submodule `kde_contours`

- BREAKING CHANGE: Change np.meshgrid indexing in
`RTDCBase.get_kde_contour` from "xy" to "ij"
- feat: new submodule `kde_contours` for computing kernel density
contour lines at specific data percentiles (60)
- fix: range for contour KDE computation did not always contain end
value (`RTDCBase.get_kde_contour`)
- fix: `positions` keyword argument in `RTDCBase.get_kde_scatter`
was not correctly scaled in the logarithmic case
- ref: cleanup and document PolygonFilter.point_in_poly
- ref: move skimage code to separate submodule "external"
- ref: drop dependency on statsmodels and move relevant code
to submodule "external"

- fix: all-zero features were treated as non-existent due to relic
from pre-0.3.3 era
- fix: correct extraction of start time from tdms format
(1h offset from local time and measurement duration offset)
- fix: correct extraction of module composition from tdms format
(replace "+" with ",")
- enh: add configuration key mapping for tdms format to simplify
conversion to hdf5 format (see ``fmt_tdms.naming``)
- enh: do not add laser info for unused lasers for tdms format
- enh: dclab-verify-dataset checks for image attribute dtype
- enh: include original software version when exporting to rtdc format