Re #9118. Remove unused code.
Re #9118. Update test to use factory in way I expect it to end up.

Obviously, this won't compile until I've refactored the class itself.
Re #9118. Remove unused inclusions of IMDDimensionFactory.
Re #9118. Refactor and greatly simplify IMDDimensionFactory.

It's no longer a class, just 3 non-member functions that create
an IMDDimension object (actually an MDHistoDimension object, as before)
given an input XML string or a Poco::XML::Element.
It no longer retains ownership of any Poco::XML object, which will allow
me to handle the previously unreleased memory issues.
Merge branch 'feature/9118_simplify_imddimensionfactory' into develop