feat(mfem): added mfem into source tree along with patch based build system

MFEM is a dependency of our code and we want to build it with our code for portability, here I add a meson subproject which fetches mfem and builds it. Because of some issues with CMake and meson I apply a patch which manually disables running tests and building examples and miniapps in mfem. This is okay as mfem here is not intenteded to be linked against other programs (though it still can be).

meson.build

@@ -3,7 +3,10 @@ project('4DSSE', 'cpp', version: '0.0.1a', default_options: ['cpp_std=c++23'])
 # Add default visibility for all C++ targets
 add_project_arguments('-fvisibility=default', language: 'cpp')
 
+# Build external dependencies
+subdir('build-config')
 
+# Build the main project
 subdir('src')
 if get_option('build_tests')
     subdir('tests')