BLXI-225: provide unit for the 'count_time' data in the detector nexus data file 

need to give default 's' unit

Task-Url: http://jira.diamond.ac.uk/browse/BLXI-225
ensure initialisation explicit
BLXI-228: pixium live image display no data after IOC restart
ensure this plugin is enabled for each scan, but could be configured to not add data to scan data file as the actual image is written by EPICS hdf plugin.
http://jira.diamond.ac.uk/browse/BLXI-228
make sure this class has options to support write detector pixel size for data reduction but without change default -currebt behaiour.