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 cell.className() is a problem since it is not in a CCellularGridND concept. But it is used at many places in IO package. Use it for now. I suppose we should instead write a wrapper function className( Type ) that falls back on Type::className() if not available.