Feature request: ASE calculator

[Andrew-S-Rosen]

This is a feature request to have an ASE calculator interface.

Copy of dftd4/tad-dftd4#114.

[marvinfriede]

I will probably only implement the ASE calculator for D4. I am considering to merge the D3 and D4 projects, because the two projects share a lot of code, and it is inconvenient to maintain them separately (as you having to post the same issue twice also shows).