Removal of Fortran API variadic functions

[corentin-prigent]

This update is motivated by problems encountered when using variadic functions, such as MMG3D_Init_mesh() in Fortran on a ARM processor.
Up to now, the Fortran API did not provide interfaces for variadic functions, as such functions do not exist in Fortran.
In this update, variadic functions from the Fortran API have been removed. They have been replaced by functions holding the same name with a different prototype, using arrays of pointer of unknown length, allowing to mimic the behaviour of C variadic functions.
As a consequence, the typical use of these functions in Fortran as been modifed. As illustrated in the libexamples, functions should be called using the following syntax:

  CALL MMG3D_Init_mesh((/MMG5_ARG_start, &
       MMG5_ARG_ppMesh,LOC(mmgMesh),MMG5_ARG_ppMet,LOC(mmgSol), &
       MMG5_ARG_end/),ier)

where variadic arguments are passed through an array of MMG5_DATA_PTR_T elements. The LOC() function must be used to pass data by reference.
The perl script generating the Fortran interface has been modified in order to have consistency between both types of elements in the array: indicators such as MMG5_ARG_start should be interpreted as INTEGER(kind=N), which correspond to the type MMG5_DATA_PTR_T.

[taupalosaurus]

Why do we need to change all these thresholds in the core sources of src/common ? That doesn't really seem desirable to me...

[corentin-prigent]

In order to validate some unitary tests, the values of these thresholds have been set ad hoc, by taking into account the truncation error of gcc. If clang is used, or if some optimizations are made, truncation errors may vary and lead to slightly different output values. That is why the threshold have been modified so that the tests are valid in these situations.