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.
There are some include tricks with eigen wrappers.  The main eigen package should be included at the end. 

> Le 5 juil. 2014 à 13:49, Jacques-Olivier Lachaud <notifications@github.com> a écrit :
> 
> In examples/dec/exampleDiscreteExteriorCalculusChladni.cpp:
> 
> > +#include <string>
> > +using namespace std;
> > +
> > +#include "DGtal/math/linalg/EigenSupport.h"
> > +#include "DGtal/dec/DiscreteExteriorCalculus.h"
> > +#include "DGtal/dec/DiscreteExteriorCalculusSolver.h"
> > +
> > +#include "DGtal/base/Common.h"
> > +#include "DGtal/helpers/StdDefs.h"
> > +#include "DGtal/io/boards/Board2D.h"
> > +#include "DGtal/math/linalg/EigenSupport.h"
> > +#include "DGtal/dec/DiscreteExteriorCalculus.h"
> > +#include "DGtal/dec/DiscreteExteriorCalculusSolver.h"
> > +using namespace DGtal;
> > +
> > +#include <Eigen/Eigenvalues>
> include of other libraries should be before DGtal includes.
> Furthermore, it would be better for examples that Eigen be hidden (in some linalg structure). I thought it was done this way, am I wrong ?
> 
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