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.
Are you sure this is mandatory?
Solvers usualy carry some data with them.
Advertising their (potential) assignability in the concept could be misleading for the user.