Populate added mass matrix terms with hydro bodies indices

[tridelat]

Right now, hydro bodies have to be added to the chrono system before any other chrono body or node. This is because the (dense) added mass matrix of hydro bodies is assumed to be at the upper left of the (sparse) global mass matrix. For some applications, it would make it easier to be able add hydro bodies after other bodies.

This could probably be done by populating the global mass matrix with added mass terms using the actual indices of the hydro bodies assigned by the system.