Add a size() function member to SDD.

It just redirects to sdd::dd::count_combinations().
Factorize optional homomorphism type.
Better text outputs for saturation_intersection and saturation_sum.
More tests.
New default configuration.
Fix saturation_intersection textual output.
Minimal export to DOT of an SDD.

Not yet implemented for hierarchical SDD.
Move lua export into tools.
Fix lua export namespace.
Using sdd::conf2 for hanoi.
Factorize flat_set configuration part.
New small example of a dictionary encoded into an SDD.
Add a tool to count the number of nodes in an SDD.

With tests.
Fix header guard.
Remove useless header inclusion.
A test was not compiled when using Cmake.
Update .gitignore.
New tool the compute the frequency of the number of arcs.
Use coherent SDD variable numbering scheme.
Add function to compute the total number of arcs from an arcs_frequency_type.