Issue 463 -- Making header files configuration files

[lsawade]

Description

This PR tracks progress towards seemless definition of floating point or double precision
computation as part of meshfem2d/-3d and specfempp.

The use can now specify

cmake ... -D ENABLE_DOUBLE_PRECISION 

during the configuration and the adjustments are propagated throughout the code.

Why?

Currently, all *.h files in meshfem2d/meshfem3d as well as the specfem_setup.hpp were hard coded. This has led to inconsistency in the binary IO (see #465).

While xmeshfem2D and xmeshfem3D are only writing double precision values to the binary files, xgenerate_databases writes base on CUSTOM_REAL that is usually defined within the configuration of SPECFEM2D/3D/3D_GLOBE.

This required quite a bit of change in the IO related functions and the cmake compilation.

What needed to change?

TODO:

• create include/specfem_setup.hpp from setup/specfem_setup.hpp.in via CMake
• create include/constants.hpp from setup/constants.hpp.in via CMake
• create fortran/meshfem2d/*.h from fortran/meshfem2d/setup*.h.in via CMake
• create fortran/meshfem3d/setup/*.h from fortran/meshfem3d/setup*.h.in via CMake
• Update src/IO/fortranio/fortran_io.cpp to read "old" mesh files correctly.
  • fortran_read_value(type_real *value, ...) indicated that the readable input is
    always dependent on specfempp's definition of type_real, when in reality meshfem2d
    only writes double precision values, and fortran_read_value(type_real *value, ...)
    reads a double and converts it type_real. Not great.
  • Solution implemented:
    • fortran_read_value(float *value, ...)
    • fortran_read_value(double *value, ...)
    • Updated the read_2d_mesh functions to explicitly read double precision values
      and then convert them into type_real after the fact.
    • Updated tests that load the database files (specfem::testing::array2d/3d)
      that invoked type_real reads to read doubles instead.

The most difficult part of this was figuring out how to create the config.h files. Part of which I'm not 100% confident whether it is required, but it works. Please see the fortran/meshfem?d/setup/config.h.in and the corresponding fortran/meshfem?d/setup/CMakeLists.txt for generation of the necessary files.

Issue Number

Closes #463
Closes #465

Checklist

Please make sure to check developer documentation on specfem docs.

• I ran the code through pre-commit to check style
• My code passes all the integration tests
• I have added sufficient unittests to test my changes
• I have added/updated documentation for the changes I am proposing
• I have updated CMakeLists to ensure my code builds
• My code builds across all platforms