BLVII-52: Created NXDetectorPlugins to read stats time series array
BLVII-57 - Created a PositionInputStreamCombiner and test

Removed MultiplePositionStreamIndexer as this can be replaced by
indexing the stream from a PositionInputStreamCombiner with the existing
PositionStreamIndexer.
BLVII-57 - Exposed gda.device.detector.nxdetector.plugin package from
core
BLVII-57 - Removed MultiplePositionStreamIndexerTest as no longer
required
BLVII-57 - Created a NullNXPlugin which does nothing.
Merge branch 'master' of ssh://dascgitolite@dasc-git.diamond.ac.uk/gda/gda-core.git