Issue214 tank hex

This pull request addresses among other updates the bug that you reported in 
https://github.com/lbl-srg/modelica-buildings/pull/210. 

I moved the discussion of this bug to https://github.com/lbl-srg/modelica-buildings/issues/214.

I closed your pull request as the development is done not on the master, but on the branch https://github.com/lbl-srg/modelica-buildings/tree/issue214_tankHex

Could you please check whether this pull request addresses the issue that you reported?
Thanks.