Add TimeSeries data-access method to read data from a directory

@stuartthebruce has suggested a new data-discovery method that is half-way between TimeSeries.read (which requires a list of files) and TimeSeries.find (which calls out to gwdatafind) which does 'here's a directory and some parameters, go find my data in there and read them'.

The use case for LIGO is point GWpy at /dev/shm{/kafka} and have it read the data from there for a given GPS interval.