Extrapolating DFT towards the complete basis set limit

Changes from v3.0:

- Fixed files that were tracked by git as symlinks instead of actual files.

Changes from v2.0:

- updated correct double-hybrid α to 2.2 from 2.4

- fixed drawing of previous extrapolation results in several figures

Changes from v1.0:

- swapped out CCSD(T) data for the L7 dataset, now using data of Al-Hamdani for the 5 species where agreement is sufficient

- redesigned most figures following reviewer comments

- moved Table 1 from SI to main paper

Changes from v0.2:

- added run scripts in a separate folder

- added README.md in external folder