Make Exercise::getThing work as advertised, and actually return the
Thing.
Return a list of category types, which makes the form handling easier.

Use the correct variable name when updating the thing element
Distinguish between the name of a unit type and it's label (eg
name="meters", label="Metres")

Fix typos in variable names
Ensure that the distance and activity elements are set correctly.
Actually implement the set/update functions and track the changes to the
normalised value.