gets a subtensor element from an MTensor.
- Used in C/C++ code in a library function called from Mathematica.
- Uses the same numbering scheme as Mathematica parts, i.e. the first element is 1.
- The array is assumed to have length at least equal to .
- A nonzero error code value will be returned if the element does not exist or if the rank of is not at least as big as .
- The destination * should either be null (0) or an MTensor allocated with MTensor_new.