Add translationUnits to Size Units values

[TomazErjavec]

For aligned parallel corpora Translation Unit would be the most natural way to give the size of the corpus. However, while the units include rather unusual Units such as phoneticUnits, it does not contain translationUnits. It would be good to have them.

[kosarko]

there is a discussion in #192 about how these units came about.

If you need a quick resolution update the value-pairs element named metashare_sizeunit in input-forms.xml . In the newest version the displayed-value is a key in messages.xml, in the older it's just the value the user sees.

[TomazErjavec]

Sorry, I know about that discussion of course, but couldn't find it before.
OK, we might go for the quick solution, rather than the slow one 👿 thanks!