up #387
 - add substructring to mesh

Conflicts:
	feel/feeldiscr/doftable.hpp
	quickstart/laplacian.cpp

(cherry picked from commit 8e8aef99bf26aff7079a035dc7ad51b416702e1a)

Conflicts:
	feel/feelfilters/gmsh.hpp
	feel/options.cpp
up #387 : add some interfaces

Conflicts:
	feel/feelalg/matrixsparse.hpp
	feel/feeldiscr/doftable.hpp

(cherry picked from commit 2ca462c623a5bd2606c623d2caf676b82c29b58c)

Conflicts:
	feel/feelalg/matrixeigendense.cpp
	feel/feeldiscr/doftable.hpp
up #387: new interface for sqrt matrix

Conflicts:
	feel/feelalg/matrixeigendense.cpp
	feel/feelalg/matrixsparse.hpp

(cherry picked from commit 6dcd7d85368d80b7beee32b5ba9295de9a124a66)

Conflicts:
	feel/feelalg/matrixeigendense.cpp