use correct degenerated flag on points + abort splitAllEdgesConnectedToSingularity if split leads to large area change in param plane

Allow Periodic constraints on extruded meshes (between the "bottom" and "top" entities, where the mesh is copied and is guaranteed to be periodic). In this case the periodic connections will be generated automatically, without having to call Periodic after meshing explicitly. Cf. #661.

Merge branch 'master' of http://gitlab.onelab.info/gmsh/gmsh

using "true" coordinates (and not special ones). Then, it seems to
behave better for singular points on non-periodic surfaces (i.e.
when the seam has been removed).

Physical is a reserved keyword of the .geo language - use PhysicalGroup instead (cf. #614 and a0d5ff7b)

Rename plugin

See merge request !281

Export edges and vertices to CAD X3D

See merge request !276

replace OS-dependent icon code with (introduced in fltk 1.3.3) Fl_Window::default_icon()

See merge request !280

detect cycles in extrusion (layers) attributes so that we can generate extruded meshes for rotationally periodic geometries

don't issue error when checking periodicity on e.g. transfinite surfaces (where the corresponding node map is empty)

test '9' shortcut for full onelab reload (Ctrl+0 seems not to be working on some Windows machines - cf. @bsalves)