- 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)