Added Lapack in linalg.h and a fixed a minor #elif.

- The other params are not documented
- note that this struct is not directly called in our application (you can find how to use these dot methods in our dot unittests). To make it work, our function [dot(a,b)](https://github.com/shogun-toolbox/shogun/blob/develop/src/shogun/mathematics/linalg/internal/modules/Redux.h#L56) _HAS_ to be able to call a method named "`compute"`. We don't want to write this function over and over again for all the backends we have. So this templated backend thingi takes care of that. You _must_ keep the signature of `compute` method same as others. just dense dot for now.