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.
If your question is 'Can i write a linalg backend compatible with the dec package using RealVector, RealPoint and SimpleMatrix?', then the answer is no.
Here are the three main reasons:
- Vector and matrix should be dynamically sizeable, which isn't true for neither RealVector nor SimpleMatrix.
- For efficient representation of operator, the backend should provide a dynamic sized sparse matrix.
- There is no linalg solver that use SimpleMatrix.