Add gc damping field to the parser

For now, there was some goal related to adding gc damping by adding friction in the joints. That seems to be a bit of a pipe dream. Till that happens, we should also support a simplified gc damping setup instead of forcing the users to implement it themselves.

Seems to be that 5 x sim-dt is a somewhat reasonable value for squishy things. And sim-dt seems to be a reasonable value for non-squishy things.