Kutalmış Berçin authored Feb 11, 2020 and Andrew Heather committed Feb 18, 2020

  - `tensor` and `tensor2D` returns complex eigenvalues/vectors
  - `symmTensor` and `symmTensor2D` returns real eigenvalues/vectors
  - adds new test routines for eigendecompositions
  - improves numerical stability by:
    - using new robust algorithms,
    - reordering the conditional branches in root-type selection