Corrected error in HumidifierPrescribed and improved variable names and documentation

Corrected error in `HumidifierPrescribed` and improved variable names and documentation.
This is for #33.
The pull request is to merge this change to issue21_media, as issue21_media diverged significantly from the master. If issue21_media is merged to the master prior to this pull request, then this change should be merged to the master.