Add another overload which automatically determines whether the rigid
body world should also be locked.

Allows us to get rid of a bunch of local variables that only existed
to force the entire vector to be loaded.

The pattern we want to use is:

```
if (isEntity())
  do entity stuff (e.g. get entity motion accessor)
```

or

```
if (!isEntity())
  return;

do entity stuff
```

That's clearer than

```
if (isSensor())
  return;

do entity stuff
```

because the fact that !isSensor() is equivalent to isEntity() is not
always immediately clear.