Matlab export: (almost) like load_gmsh2 converter

The Matlab exporter can now exports:

• Every type of elements (triangles, quads, ...)
• Follows the converter file in utils/load_gmsh2.m

Some differences/errors still exist:

• The matlab output do not contain an array with every elements (they are separated according to their type)
• When "saveall" is selected, the elementary tag is not applied (=0), but I don't know why.