Initial support for quadrilateral meshes

What is included?
 * Utility quadrilateral mesh creation
    * Boundary subdomains are supported
 * Cell integrals
    * Higher-order CG
    * HDiv and HCurl elements (manual assembly required, just as for extruded meshes)
 * Test cases for the above
 * Visualisation output

What does not work yet?
 * Facet integrals
 * Non-affine mesh elements
 * Extruded quadrilateral meshes
 * Immersed quadrilateral meshes (not tested)

I'm not sure if that's feasible. The triangle version simply uses the BDM function space, while for quadrilaterals one has to assemble the HDiv function space by hand, just as for extruded intervals.