i–PI force evaluation method & Nix building

This PR adds two features:

• A new method in FORCE_EVAL called IPI.
• A set of files that allow CP2K to be built using the Nix package manager, contributed by @sheepforce

The motivation for these changes is to open up the power of CP2K as a MD/Optimization engine to other software, so that other programs may take advantage of the advanced features of CP2K using their own force evaluation methods. The original goal was to interface our multiscale and fragment calculation package Spicy with CP2K. However, since communication takes place through the communications protocol established by i–PI, any code that can communicate this way could be interfaced to CP2K with out further modifications (or so we hope).

i–PI protocol & implementation

i–PI itself is an engine built for path integral molecular dynamics build on a "server–client" division of labour. i–PI takes the role of the server and handles the evolution of the system, while one or more clients are responsible for the evaluation of the atomic forces. The server communicates with the clients through network sockets, sending atomic positions and cell data and receiving forces in turn. Here, the communications protocol (documented mostly by implementation…) established by i–PI is reused to provide as universal an interface as possible.

CP2K has previously implemented i–PI client side functionality (as a runtype), meaning that it has been able to compute forces for an i–PI server. Here, the other side of the coin is implemented, where an external program provides forces for CP2K. Existing infrastructure is reused where possible.

Example input:

&GLOBAL
  RUN_TYPE MD
&END GLOBAL

&FORCE_EVAL
  METHOD IPI
  &SUBSYS
    &CELL
      ABC 10.0 10.0 10.0
      PERIODIC NONE
    &END CELL
    &TOPOLOGY
      COORD_FILE_FORMAT xyz
      COORD_FILE_NAME yourMoleculeHere.xyz
    &END TOPOLOGY
  &END SUBSYS
&END FORCE_EVAL

&MOTION
  &MD
    STEPS 1000
    TIMESTEP 0.25
    ENSEMBLE NVT
    TEMPERATURE 300
    &THERMOSTAT
      TYPE CSVR
      REGION MASSIVE
      &CSVR
        TIMECON 25
      &END CSVR
    &END THERMOSTAT
  &END MD
  &DRIVER
    HOST myHost
    PORT 8421
    UNIX
    SLEEP_TIME 0.1
  &END DRIVER
&END MOTION

Note the DRIVER subsection, which gives the precise socket location and type.

Nix

Nix is a package manager and build tool focused on transferability and reproducibility. The files added (flake.*, nix/overlay.nix, nix/cp2k.nix) provide a reproducible build and development environment for CP2K without interfering with any existing infrastructure.

Usage is simple:

• Install Nix
• nix build will build/download all dependencies and build CP2K
• nix develop will enter a development environment with all dependencies, fortls, and fprettify

Possible issues

• This is my first time working on production–level Fortran code, so please review carefully.
• I'm not quite sure how what has been implemented interacts with CP2Ks parallelism. In practice, I don't think parallelism is required for an i–PI server, but it would be good if behaviour was correct anyway.
• This implementation is limited to a single client.