Extended PatternPresentingCommands to support multiple durations

PatternPresentingCommands now take a list of durations, which specify
when to record a measurement. The FeatureResponses class and subclasses
can now also return measurements for each duration. .ty files have
been changed to respect this new way of setting the default duration
of a measurement.

Legacy code has also been extended to convert old curve_dicts into
the new nested NdMapping data structure. Finally class attributes
are now either deleted, moved or converted when loading a snapshot.