Refs #7940 - Allow creation of "Guide" objects.

* Converts guide to hexahedron and uses pre-existing code to create shape.
* Updated schema.
* Added unit tests.
Merge branch 'master' into feature/7940_guide_idf_shape

Refs #7940

Conflicts:
	Code/Mantid/instrument/Schema/IDFSchema.xsd
Refs #7940 - Change name from guide to tapered guide.
Refs #7940 - Utilise getOptionalShape when building tapered-guide.

And do some tidying up around the use of anonymous DEFAULT_x constants
elsewhere in the recent changes to ShapeFactory.
Refs #7940 - Centre is now defined as middle of start aperture.
Refs #7940 - Slight fix; rotate first, then translate.
Merge remote-tracking branch 'origin/feature/7940_guide_idf_shape'