Fix pyramid element ref coords

[sandro-elsweijer]

Describe your changes here:
The implementation of pyramid element reference coordinates was faulty:

This can only be observed, if the pyramids are written out as curved and if the nonlinear subdivision level in ParaView is >0.
After the fix, it looks like this:

Here is a quick code for generating pyramids and write them as curved elements:

#include <t8.h>
#include <t8_cmesh/t8_cmesh_examples.h>
#include <t8_forest/t8_forest_general.h>
#include <t8_forest/t8_forest_geometrical.h>
#include <t8_forest/t8_forest_profiling.h>
#include <t8_forest/t8_forest_io.h>
#include <t8_forest/t8_forest_vtk.h>
#include <t8_schemes/t8_default/t8_default_cxx.hxx>

int main(int argc, char **argv)
{ 
  int mpiret = sc_MPI_Init (&argc, &argv);
  SC_CHECK_MPI (mpiret);

  sc_init (sc_MPI_COMM_WORLD, 1, 1, NULL, SC_LP_ESSENTIAL);
  t8_init (SC_LP_DEFAULT);
  t8_cmesh_t cmesh = t8_cmesh_new_hypercube(T8_ECLASS_PYRAMID, sc_MPI_COMM_WORLD, 0, 0, 0);
  t8_forest_t forest =  t8_forest_new_uniform (cmesh, t8_scheme_new_default_cxx(), 1, 0, sc_MPI_COMM_WORLD);
  t8_forest_write_vtk_ext(forest, "testfile", 1, 1, 1, 1, 0, 1, 0, 0, NULL);
  t8_forest_unref(&forest);
  sc_finalize ();

  mpiret = sc_MPI_Finalize ();
  SC_CHECK_MPI (mpiret);
}

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As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

General

• The reviewer executed the new code features at least once and checked the results manually

• The code follows the t8code coding guidelines

• New source/header files are properly added to the Makefiles

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Tests

• The code is covered in an existing or new test case using Google Test

Github action

• The code compiles without warning in debugging and release mode, with and without MPI (this should be executed automatically in a github action)

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