Add extensible field implementation and other misc updates

Add the extensible fields implementation for the GFunctions, adding
addGFunction(), removeGFunction(), removeAllGFunctions(), and
gFunctions() methods.

Add \max-fields and \reference to idd file.

Remove NumberofDataPairsoftheGFunction field and implementation since
this can be determined from the extensible groups in the forward
translator.

Remove Q_PROPERTY from Impl.
Change BOOST_ASSERT to OS_ASSERT.

Add more unit testing.
Add object to hvac_library.osm.
Add object to OpenStudioPolicy.xml.
Add Reverse Translate, commented out

Adds reverse translate for GroundHeatExchanger:Vertical, however, in
ReverseTranslator::translateAndMapWorkspaceObject() the code to call
the translation is commented out.

The implementation works when uncommented in manual testing.