Restore broken library link for desalination case

[jtgrasb]

This PR restores the broken PTO library link for the desalination case by moving the inertia block outside of the PTO and extracting the velocity within the Piston & RO subsystem. See also updated degenerate mass docs in WEC-Sim PR#1595.

While investigating the degenerate mass issues, I found that the inertia block could be moved outside the PTO block itself while still avoiding the degenerate mass errors. This does change the location of the mass depending on the PTO location and initial displacement, but this effect is small for a small mass. For small mass values (which should be used for resolving degenerate mass anyway), the difference in results is negligible between locating the inertia block inside vs. outside the PTO:

[akeeste]

Thanks @jtgrasb This is working for me. Approved and merging!