Introducing mixed PLIC/PPIC capabilities in nga2

[fabienevrard]

$\textbf{\color{#000000}{Content of the pull request}}$

• In the vfs_class:
  • Replacement of all references to PlanarSep_type (the Fortran version of the IRL PlanarSeparator class) by the new SeparatorVariant_type (the Fortran version of the IRL SeparatorVariant class). This enables VOF advection using a mixed PLIC/PPIC reconstruction.
  • New interface reconstruction function called build_jibben, which uses Jibben's parabolic reconstruction algorithm to reconstruct a PPIC interface.
• A new I/O class that uses the VTK XML file formats:
  • The grid data is written as a list of .vtr files (i.e., VTK's RectilinearGrid format).
  • The surface data is written as a list of .vtu files (i.e., VTK's UnstructuredGrid format). This supports the new VTK_BEZIER_TRIANGLE cell type, enabling the cheap approximation of PPIC surfaces. The surface can be refined directly within Paraview by increasing the Nonlinear Subdivision Level of the PLIC object.
  • The particle data is written as a list of .vtp files (i.e., VTK's PolyData format).
  • These data files are collected into .pvd header files, a Paraview-specific file format that enables their time collection/synchronization.
• A new example called drop_translation that uses Jibben's algorithm to reconstruct a PPIC interface, as well as the new vtk_class to output all data.

$\textbf{\color{#f5750e}{Important compilation information}}$

In order to compile, the code needs to be linked to this new version of IRL: https://github.com/cmfr-ae-illinois/interface-reconstruction-library/tree/variant

Note that this version of IRL requires GNU's QuadMath library. On MacOS, this requires compiling IRL using the GCC C/C++ compilers (clang will fail).