Dftd4

Complete rewrite of DFT-D4 to get this project in a more sustainable shape. This change introduces API breakage (Fortran, C and Python API), therefore leads to a major version bump.

- improve unit testing and more testcases
- cleaner Fortran API via one single module
- more robust C API with `void*` based Fortran objects
- CFFI based Python API instead of ctypes based interface
- fixes issues with Intel 19
- uses [mctc-lib](https://github.com/grimme-lab/mctc-lib) as geometry backend
- remove EEQ implementation and create separate project
- uses [multicharge](https://github.com/grimme-lab/multicharge) library for EEQ model
- more stable choice of Ewald parameter (results for periodic calculations might differ slightly)

- parameters for DFTB
- parameters for revDSD double hybrid functionals
- serialization of calculation results to JSON or TOML

- Bugfix: i->i' contribution is calculated correctly :beetle:

- Python: API and ASE-wrapper separated
- Bugfix: read `.CHRG` file correctly
- Bugfix: stress tensor is calculated correctly
- Fortran mod-files are now installed with `ninja install`

- Bugfix: argparser logic :beetle:
- Bugfix: distance calculator for PBC had wrong indices for ii' case :beetle:
- Compatibility: OSX build enabled
- Compatibility: C-API changes
- Compatibility: Python 3.5 support
- Wrap file for dependency management with `meson`
- `openblas` accepted as `la_backend`-option, custom backend enabled for build

dftd4` version for molecular and 3D periodic systems. This version includes a reworked C-API and a new Python wrapper.