Block use of sim/multiplayer datarefs

[tngreene]

sim/multiplayer/* datarefs are bad bad bad. We shouldn't allow the 2.7x to add them to it's dataref lookup dictionary, and we should emit an error and tell the person to choose something else (maybe we can suggest an alternative?)

• [ ] Block addition in xplane_datarefs_txt_parser

• [ ] Emit error in XPlaneObject.collectAnimAttributes

• [ ] Unit test showing that xplane_datarefs_txt_parser no long has it

• [ ] Unit test showing error emitted

We'll doc this in the release notes. Users won't notice something hidden and the error clearly states what's going on. Plus, there is this record here.

[kbrandwijk]

Regarding 'block addition', should they simply be skipped when reading datarefs.txt, or should an explicit error be thrown?

[tngreene]

Since sim/multiplayer will (probably) always be in Datarefs.txt, we can't give people an error during the file loading. We'll just ignore it.

If someone is using it as a dataref, we need to give a full on error in the logger and force them to choose a different one.