Clarify nonzero mass/inertia message and behavior.

Running this code:

```cpp
Model model;
State state = model.initSystem();
model.printDetailedInfo(state, std::cout);
```
produces an unnecessary warning message:

```
MODEL: 

ANALYSES (0)

BODIES (1)
body[0] = ground (mass: 0)Body 'ground' is massless but nonzero inertia provided. Iner
tia reset to zero. Otherwise provide nonzero mass.
 (inertia: [~[0,0,0]  ~[0,0,0]])

ACTUATORS (0)
numStates = 0
numCoordinates = 0
numSpeeds = 0
numActuators = 0
numBodies = 1
numConstraints = 0
numProbes = 0

STATES (0)
```

[Here](https://github.com/opensim-org/opensim-core/blob/20beb87ef317c69a4a1d11d4f12d18dcfc0e68ae/OpenSim/Simulation/SimbodyEngine/Body.cpp#L177) is where the message is generated. 

This PR only emits that message if the inertia property was indeed set to nonzero, and also modifies the inertia property.
property is a Vec6 and _inertia is the Inertia tensor.