Draft Pixi build configuration

[avcopan]

When working, this will allow one to build a MESS into a conda package using pixi as follows:

pixi build

This can then be uploaded to a given conda channel using rattler-build as follows:

export ANACONDA_API_KEY=<anaconda.org API key>
rattler-build upload anaconda -o <channel name>

Currently, it installs all of the necessary executables and libraries, and the messpf executable works when tested against the example. However, the mess executable aborts with a memory error:

terminate called after throwing an instance of 'std::bad_array_new_length'
  what():  std::bad_array_new_length
Aborted (core dumped)