ENH: improve funcs and opers in Tensor types

  - 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