Seb/add 2decomp iface CMakeLists

[cayrols]

This Pull Request addresses the creation of the 2decomp&FFT library interface.

We create a CMakeLists.txt file, that downloads the source and compiles the library.
The created library and the header file will be located in the build folder.

Note: We remove 2decomp as a submodule.

[cayrols]

UPDATE:
I integrated the call to this CMakeLists.txt into the main one so that it builds the interface.

On Saturn, it works with the following modules fftw/3.3.10/gcc-7.3.0-ujnq cmake/3.22.2/gcc-7.3.0-wes6 and a classic install to 2decomp.

Then, the compilation from the root of fiber:

mkdir build
cd build
make -DMPI_DIR=$ICL_OPENMPI_ROOT -DFIBER_ENABLE_2DECOMP=ON -DFFTW_PATH=$ICL_FFTW_ROOT -DDECOMPFFT_ROOT=~/workspace/2decomp_fft-1.5.847 -DFIBER_FFT_LIB_DIRS="~/workspace/2decomp_fft-1.5.847/lib;${ICL_FFTW_ROOT}/lib;${ICL_OPENMPI_ROOT}/lib" -DFIBER_FFT_INCLUDE_DIRS="~/workspace/2decomp_fft-1.5.847/include;${ICL_FFTW_ROOT}/include" ..

Then, as stated here, using these modifications the execution works:

mpirun -n 2 ./benchmarks/test3D_CPU_C2C decomp2d

Feel free to comment this.