Use //link/inertial/density element in parser

[scpeters]

Desired behavior

After describing the //collision/density element, the Proposal for Automatic Moments of Inertia Calculations states: "A //link/inertial/density element would also be added in the spec to allow users to specify the density values on a link level instead of specifying the same values for each collision."

This element should be was added to the spec in #1299 but still needs to be used in the parser.

Alternatives considered

Implementation suggestion

Additional context

[scpeters]

see #1335