Dclab

- enh: dclab-verify-dataset now also checks whether the sheath and
sample flow rates add up to the channel flow rate
- ref: Configuration does not anymore load unknown meta data
keyword arguments, but ignores them. This implies that
dclab-verify-dataset will not anymore check them actively.
Instead, any warning issued when opening a file is added
to the list of cues.
- setup: bump nptdms to 0.23.0

- fix: data export to HDF5 did not work when the "frame rate"
is not given in the configuration

- enh: add checks for valid keys in the Configuration dictionary
of a dataset `RTDCBase().config`; unknown keys will issue
an UnknownConfigurationKeyWarning (58)
- ref: moved `rtdc_dataset.fmt_hdf5.UnknownKeyWarning` to
`rtdc_dataset.config.UnknownConfigurationKeyWarning`
- ref: renamed `rtdc_dataset.config.CaseInsensitiveDict`
to `rtdc_dataset.config.ConfigurationDict` and added option
to check new keys

- fix: disable computation of Young's modulus for reservoir
measurements (75)
- enh: new keyword argument `req_func` for `AncillaryFeature` to
define additional logic for checking whether a feature is
available for a given instance of `RTDCBase`.

- enh: add `data` property to ICues (and use it when checking for
compression)

- fix: when computing the contour from the mask image, always use
the longest contour - critical when the mask image contains
artefacts
- fix: minor issue with dclab-verify-dataset when nptdms was not
installed and an exception occured
- enh: dclab-verify-dataset shows some info on data compression