Integrating 2decomp C interface

[G-Ragghianti]

I have imported the 2decomp c interface code into my branch at https://github.com/icl-utk-edu/fiber/tree/gragghia/2decomp

I think that we need to add cmake support to reproduce what the make is doing (external/2decompFFT_c_iface/Makefile) to compile lib2decomp_fft_iface.a:

gfortran   -o build/decomp_2d_ciface.o -I /home/gragghia/spack/opt/spack/linux-centos7-broadwell/gcc-9.2.0/2decomp-1.5.847-doqto43oprkxzqw36pd2nrgagvbvqsg6/include -c decomp_2d_ciface.F90
ar vr ./lib/lib2decomp_fft_iface.a build/decomp_2d_ciface.o
r - build/decomp_2d_ciface.o
ranlib ./lib/lib2decomp_fft_iface.a

Then we need the main fiber CMakeLists.txt to link to lib2decomp_fft_iface.a.

Should the build of the library be done in a separate CMakeLists.txt within external/2decompFFT_c_iface/ or can this be done in the fiber CMakeLists.txt? Help is appreciated.