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.
Dimension is defined from axioms of vector spaces.
I don't think this is revelant in this concept.
Moreover dimension could be infinite or unknow and one could still check the vector space structure.