get_segments: allow compound flag names

    - e.g. flag1&flag2 will download each flag individually, and return the intersection
    - allows '&': intersection, '|': union, and '!': difference
    - probably not very robust under weird user input
SummaryState: moved compound flag name parsing to segments.py
SummaryTab: fixed minor bug
SummaryTab: minor update to plot option parsing
SegmentPlot: fixed GPS extent and flag labels