In their equilibrium position, the principal inertial axes of a body are not necessarily aligned with WEC-Sim's global coordinate system. Allowing products of inertia to be defined would make the inertia calculation much easier for users with highly irregular, asymmetric or angled bodies. Adding this feature involves:

• adding a body class property inertiaProducts and linking it to Simscape in the body library
• adding inertiaProducts to the added mass adjustments
• documenting the proper convention for products of inertia.

I chose to match Simscape's convention for the product of inertia calculation. Some CAD, like Solidworks, use a negated convention.

This addresses project card https://github.com/WEC-Sim/WEC-Sim/projects/58#card-86097325 and issue #952