Hierarchy of tags for test division

Improve tagging to introduce a hierarchical tree of sub tags.
e.g. in the case of test cases relating to an application A, which has components B, C and D, and D itself a component E, have the tags:
A
A.B
A.C
A.D
A.D.E

split with the delimiter '.' in this case. Delimiter up for discussion!

This will prevent the tag field being littered with a mess of ungrouped tags, and also allow all of the above group to be tested when the tag 'A' is given.

These could be supplemented by other tags, such that if tags A and X are specified, all those in the group A and also matching X are run (e.g. X could be all single processor runs, or all those on P1DG-P2, ...)

This type of functionality was hinted at previously when I added (and proposed merging) an option in the testharness to search on substrings in the test name. This is more robust and just a nicer idea.

Specific example:
iceshelf
iceshelf.interfacepressure
iceshelf.interfacepressure.freesurface_scaling
iceshelf.interfacepressure.perturbation_density
iceshelf.thermodynamics