LibMBD interface

Dear All,

We are opening this MR to check in an interface for the many-body dispersion (MBD) van der Waals correction as a follow-up of the discussion of a recent thread in the developer mailing list (https://lists.quantum-espresso.org/pipermail/developers/2020-November/002351.html). In this MR, you will find a non-CMake build interface (currently serial and can be extended to use scaLAPACK) as well as a CMake version kindly provided by Daniele Cesarini; these interfaces connect to a standalone MBD library (https://github.com/jhrmnn/libmbd/) that is also already used in multiple ab initio packages (e.g., FHI-AIMS and DFTB+).

This MR (the first step of a series of follow-up MRs) contains an interface that allows one to perform scf calculations using PBE+MBD, PBE0+MBD, and HSE+MBD. This is accompanied by a simple test case in the test-suite test-suite/pw_vdw/vdw-mbd.in and update in the PW documentation page.

In the next set of MRs, we will include the MBD contributions to the ionic forces and stress tensor. We are also working on bringing in the scaLAPACK interface for this MBD library to improve the computational performance.

Best, Szabi, Hsin-Yu (@kosinyj), Rob (@robdistasio), Alexandre