simplified labeling: uses now mouse buttons to add / remove observation dates from the date collection

recognizes observation date from Landsat scene ID in file names

ImageChip context menu offers:
- to load the respective scene in QGIS
- to copy the image chip to the clipboard

changed BandView widgets

new capabilities:
save and load text file based definitions of time series data files
supports multiple sensors in a time series

scrollwindow focusses on specified date

deal with undefined no nodata values