* Draft of a eigenvalues solver (does not support complex and does not re-use the QR decomposition)

* Rewrite the cache friendly product to have only one instance per scalar type!
This significantly speeds up compilation time and reduces executable size.
The current drawback is that some trivial expressions might be evaluated like conjugate or negate.

* Renamed "cache optimal" to "cache friendly"

* Added the ability to directly access matrix data of some expressions via:
- the stride()/_stride() methods
- DirectAccessBit flag (replace ReferencableBit)