Put all detail in a tensor sub-folder

[coder3101]

Most of Boost Libraries have a separate subfolder named detail which contains mostly detail and implementation of algorithm and contains detail namespace.

In the current tensor extension the detail namespace are mangled into public uBLAS namespace. This is bad from abstraction point of view and also maintainability and readability.

We should reorganize our files so that we have a structure that is same as most Boost Libraries.