plotter.ticks: removed class that isn't used
plotter.ticks: overhaul of auto-tick locator

    - fixed auto-location of ticks for better time-formatting
    - fixed a number of other minor issues (unused imports, etc)
Histogram{Axes,Plot}: reworked histograms

    - standardised Axes and Plot objects
    - special case HistogramAxes.plot method with Series and Table handles
plotter.ticks: minor reworking of auto-scale
ticks.AutoTimeLocator: fix bug in scale setting
table.utils: moved table/row getters into new mod
table: updated for LIGO_LW tables only

    - removed refernces to astropy.table
    - added rate.py module to calculate rate TimeSeries from ligolw.Table