Switch to a simple static member function to implement singletons, which is guaranteed to be thread-safe in C++11.

Fix XML output when range flag used

See merge request !761

Fix some issues with revised Mesh::MakeOrder routine to address failing tests

See merge request !762

Fix/interppointdatatofld

See merge request !754

feature/dump-mesh-on-failure

See merge request !756

fix/gmsh-writer

See merge request !747

Merge branch 'feature/dump-mesh-on-failure' of gitlab.nektar.info:meshing/nektar into feature/dump-mesh-on-failure

Remove unnecessary consts from integer return types to avoid icc warnings

This MR removes unnecessary consts from integer return types to avoid many, many warnings from the Intel compiler.

See merge request !742

Conflicts:
	utilities/NekMesh/InputModules/InputMCF.cpp

fix/cmake-doc

Turning developer guide into a git submodule recently broke cmake configuration for docs in master.
This fixes that by copying the logic of tutorials submodule.

See merge request !752

2D Periodic Boundary Conditions

Adds the ability to  make pairs of curves periodic

See merge request !733

Add developer-guide submodule

The MR adds the developer-guide repository as a sub-module to the main Nektar++ repository.

See merge request !751