performance optimization: gmaps_length_get_units()

Hi,

I recognizes a deficit in performance for function gmaps_length_get_units(), if you have an multilangual side, which run's not in default language.
The translation of all the length units at each page load costs noticeable time, approx. 40ms per page load.

To overcome this, I created a patch, who caches all the translations for length units as well. Cached translations get reseted via input-parameter like done for the unit-array as well.