Streamlined featuremapper return interface

Return values of measurement commands are now always indexed by duration, rather than both time and duration. Only in the global dictionary are measurements indexed by time and duration. Additionally all collections of SheetView objects are now SheetStack objects. Additionally update_activity and update_sheet_activity are now only used to update the GUI activity window and will therefore store their results to .views.maps._activity_buffer on each sheet. Actual Activity measurements should be carried out using measure_activity.
pattern_present handles activity SheetViews differently

Activity sheet views are now only updated when using pattern_present if the GUI Activity window is opened.
Updated Activity plotgroup + return types of measure_activity
Fixed passing of duration in testpattern window
Updated model editor to find activity in _activity_buffer
Fixed selectivity_multiplier passing bug in featureresponses
Updated imagen submodule reference