Merge pull request #26 from wtsi-npg/devel

pull changes from wtsi devel to my devel
switch to samplesheet driver if NPG_CACHED_SAMPLESHEET_FILE is set; samplesheet driver - ensure that spiked_phix_tag_index is defined for a pool
pulled changes from wtsi-devel
Merge remote-tracking branch 'mg8-remote/devel' into devel

Conflicts:
	Changes