Make ScannerClient.series_info return a dict for a single series

Previously this filled in an dict for each series in an exam dir,
but this method didn't appear to get called anywhere else in the
codebase. Instead, this basic information was getting computed for
individual series elsewhere straight from the dicoms. This way
seems much cleaner (only have one method that gets useful summary
information about a series) while not being overly expensive (by
computing info about all of them when you just care about one series
at any given moment).