dual index refinements

ensure max length of "Index" used by MiSeq is the split dual indexs' size,
and use custom default_expected_tag in preference to Index{,2} when using
samplesheet as source of LIMS info
Merge remote-tracking branch 'wtsi-npg/devel' into dual_index

Conflicts:
	Changes
Merge pull request #175 from dkj/dual_index

Dual index - refinements