Fixed typo.

This commit removes in all models the typo “the the”.
Updated main info section and user guide.

This is for #108
Merge pull request #109 from iea-annex60/issue108_userGuide

Updated main info section and user guide.
Changed density to use default medium states and made mFactor final
Merge branch 'master' into issue102_corrFacMixVol
Added revision notes for #102
Updated comment