BufferCore: use lists of data for storage

    - allows for generic gaps in fetched data
monitors: update for lists in buffer
BufferCore: a few bug fixes and improvements
TimeSeriesBuffer: simple subclass

    - TODO: needs docstring
SpectrogramBuffer: simple extension of DataBuffer
nds: changed some logging levels
backfill: removed erroneous argument
DataMonitor.backfill: only pad for NDS>1
NDSDataIterator: manually append and pad to data
Merge branch 'buffer_devel'

Conflicts:
	gwpy/toolkits/dataviewer/timeseries.py