[BUG] NanoVDB: InternalNode::ValueOnIterator::getOrigin() doesn't compile

### Environment
**Operating System:** Ubuntu 20.04 (WSL2 in Windows 10)
**Version / Commit SHA:** openvdb 10.0.0
**Other:** Open-source OneAPI (clang)

### Describe the bug
InternalNode::ValueOnIterator::getOrigin() won't compile.

"no viable conversion from returned value of type 'void' to function return type 'CoordType' (aka 'nanovdb::Coord')"

getOrigin() is meant to return a CoordType, but mParent->localToGlobalCoord() returns void.

Same issue will likely happen for ValueIterator.

The compiler is separately throwing an error about not being able to bind a temporary of type uint32_t (from BaseT::pos()) to an lvalue reference of type Coord (the Coord& input in localToGlobalCoord())

### To Reproduce
Steps to reproduce the behavior:
1. Call ValueOnIterator::getOrigin() (from the unit test even I'm guessing)

### Expected behavior
getOrigin() should return a Coord

### Additional context
These getOrigin() functions probably meant to call offsetToGlobalCoord() perhaps?

Start by locating NanoVDB's InternalNode::ValueOnIterator::getOrigin() and the corresponding ValueIterator implementation mentioned in the report. Inspect how localToGlobalCoord(), offsetToGlobalCoord(), and BaseT::pos() are used, then reproduce the reported compile errors with the relevant unit test. Done means both getOrigin() functions compile and return a Coord as expected.

Written by the indexing model from the issue text.