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.
The complex is composed of three components:
- Top and bottom rows are 1D structure with different boundary conditions.
- The middle ring has different boundary conditions on its inner and outter border.
If no dangling 1-cells are present, Neumann boundary condition are used.
If dangling 1-cells are present, Dirichlet boundary condition are used.

Do you the complex should be simpler?
Should top and bottom strips be removed?