ENH: add 2D det() / inv() methods for Tensor/SymmTensor (#2724)

- 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.