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.
Why is this line changing `_inertia` rather than using `upd_inertia()` as you did above? Also, indent looks wrong on GitHub.