Calculate shelf HightUnits from device Height if missing. Don't show devices that create invalid shelves in config

[julianstirling]

Pretty self explanatory from title.

• If HeightUnits isn't set we calculate the it from the height of the unit and the hole spacing.
• If neither Height or Height units is set then the device isn't listed in the server interface

Relates to #82 - Closes first 2 bullets. Doesn't close the others about special shelves.

[julianstirling]

Hi @jmwright, This is ready for review. It's a small self-contained PR fixing the problem of devices with unbuildable shelves being shown in the interface.

[jmwright]

Thanks @julianstirling