V2 project (hack_DC_TGV branch): use of 2DECOMP&FFT as a sub-project

[pbartholomew08]

The idea here is to have the 2DECOMP&FFT files in a separate directory as the Xcompact3d files. At the moment, there is a 2decomp directory inside the x3div project.

The aim is to have a dedicated 2DECOMP&FFT project, under the Xcompact3d umbrella.

It will allow users to raise dedicated queries/issues for the 2DECOMP&FFT library and will allow its use outside Xcompact3d.

The library should be compiled independently of Xcompact3d and use as an external library; Makefile and cmake in Xcompact3d will have to be modified accordingly.

IMPORTANT: the 2decomp files in x3div are not up to date so we will need to start with the 2decomp files from incompact3d