avoid coverting the data of a deck item to SI multiple times

This resulted in _quadratic_ complexity if data points where retrieved
one-by-one. For the Norne case, this had the consequence that
retrieving the data for ZCORN (-> about 1M data points) took hours...
Merge pull request #91 from andlaus/only_convert_to_si_once

avoid coverting the data of a deck item to SI multiple times