caveats: in order to cleanly handle the entry panel, valve, and sensor
I had to rotate the valve and sensor models 90 degrees
so that their in-/outlet pipes point the same direction as the
entry panel.
This also enables proper handling of a valve or sensor turned vertically.
Some objects have rotation disabled entirely (as flipping them over/around makes
no sense)
When a valve is rotated, it is turned off automatically, to work around a glitch in
the rotation code.
Vanessa Ezekowitz