Lapack Compatibility with Real type.

This small merge request adds a transparent conversion layer between Real↔double to be able to call external lapack library. Of course these should later be replaced with native Eigen code. Because right now yade can calculate everything in high precision, and converting it to low precision double leads to low precision results from LAPACK.

However reimplementing those in Eigen is a separate work to be done (I can't do everything at once! :) ). This is only to help with the transition for people who use Lapack, so that you have a code which you can test the results against when you will want to start using high precision capabilities in yade.