Higher-order paraview/vtk output

[stefanhenneking]

Add support for higher-order paraview/vtk output data (geometry mesh and solution) in addition to the existing linear output on uniformly h-refined geometry meshes.

[stefanhenneking]

• Second-order vis added in Adding Second Order Visualization #84 by @ac1512 and @jbadger95 (XDMF/vtk)
• VTU-only export added in Paraview export: VTU output format and support for second-order mixed meshes #121 (enabling higher-order mixed meshes not supported by xdmf) by @ac1512

These features are enabled/disabled in module paraview via global variables:

• SECOND_ORDER_VIS
• VIS_VTU

The linear upscaling via module paraview: VLEVEL is still supported but only if SECOND_ORDER_VIS = .false.

Both XDMF and VTU support all element types. However, when SECOND_ORDER_VIS = .true. (higher-order geometry output), then only VTU supports mixed meshes.