- `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
- ensures each Tensor-container operates for the following base types:
- floatScalar
- doubleScalar
- complex
- adds/improves test applications for each container and base type:
- constructors
- member functions
- global functions
- global operators
- misc:
- silently removes `invariantIII()` for `tensor2D` and `symmTensor2D`
since the 3rd invariant does not exist for 2x2 matrices
- fixes `invariantII()` algorithm for `tensor2D` and `symmTensor2D`
- adds `Cmpt` multiplication to `Vector2D` and `Vector`
- adds missing access funcs for symmetric containers
- improves func/header documentations
