Turn the concrete Unit classes into singletons

[Russell Taylor]

With the introduction of the axis class, Unit (and it's subclasses) do not hold any member data. We should not, therefore, be creating multiple instances of concrete units to attach to the various axes of various workspaces. Instead, the Unit Factory should just hand out a pointer to the same instance every time a unit is created. Despite the title of this ticket, this should probably be achieved by modifying the factory to simply hand out the address of a concrete unit if it's previously been created, rather than by modifying the unit classes themselves.

[Russell Taylor]

(In [1136]) UnitFactory now only creates a single instance of each unit and hands out a pointer to that instance each time the unit is requested. Re #153.

[Stuart Campbell]

This ticket has been transferred to github issue ​1001