up #387 : deduce mesh size from nx and ny

(cherry picked from commit 21755fb5b67b7c55c5145dc553ab5e6220e603c5)

Conflicts:
	feel/feelfilters/gmsh.hpp
fix compilation
up #387 : added worldcomm
up #387: Add symmetry tests for sparse matrices

(cherry picked from commit 5e562e7a2701df0639ce633d55b74639e0d9683b)

Conflicts:
	feel/feelalg/matrixsparse.hpp
Merge branch 'merging-with-explicit' of https://github.com/feelpp/feelpp into merging-with-explicit