We allow Label units to regonise themeselves as QUnits. I don't like this a lot, but ConvertToMD returns scaled A^-1 units, that we'd have to write specific subtypes of MDUnit for.

Aspect ratios should be locked for Q dimensions. If the x and y dimensions of the plot are both Q dimensions, we lock the aspect ratios. Otherwise we don't. Need to add menus for this feature.

Had to fix a problem with the regex. boost::regex_match matches the entire string. Used boost::regex_search instead.

Introduce menus for selecting/unselecting fixed aspect ratios.

For older version of boost, optional type arguments must be copyable. unique_ptr is not.

Update standard chains as well. All factories tested, as well as new standard chain.

Use the factory chain wherever possible. This means that we no longer need to hard-code a type of LabelUnit in one of the constructors for the MDHistoDimension. This makes things very flexible.