- for cases where a 3D tensor is being used to represent 2D content,
the determinant is zero. Can use inv2D(excludeDirection) to compensate
and invert as if it were only 2D.
ENH: consistent definitions for magSqr of symmTensors, diagSqr() norm
COMP: return scalar not component type for magSqr
- had inconsistent definitions with SymmTensor returning the component
type and Tensor returning scalar. Only evident with complex.