Construct ProcessingInstructions correctly.

The ProcessingInstructions field was being constructed incorrectly.
Ranges were being specified using a comma, when the correct notation is
to use a colon. This patch amends this error, and also takes care of the
case where the start and end of the range are the same.

Refs #10004
Merge branch 'feature/10004_reflectometryreductiononeauto' into develop