DEC: Discrete Exterior Calculus

This new package adds discrete exterior calculus features to DGtal. It contains:
 
   - new `dec` packages (dec operators, test files, examples, documentation)
   - new `linalg` with linear algebra concepts and backends (Eigen at this point) 

As you would see, this PR, mostly managed by @elcerdo,  is quite huge and some documentation pages are still "work-in-progress". I send a PR just to start including this great package into DGtal and to clean things up thanks to your comments.
As david mentionned, there are some eigen tricks to customize matrix and vector api.
It use eigen plugin system (EIGEN_DENSEBASE_PLUGIN and EIGEN_SPARSEMATRIX_PLUGIN) and therefore EigenSupport.h should be included before *any* other eigen include. "using namespace" moved below include.