Spack related modifications

This MR aims at implementing minor changes to answer concerns that were raised in the Melissa spack package PR.

Note: the PR review also raised multiple difficulties:

• there is no Spack package for plotext which was hence moved to the dev dependencies,
• there is no Spack package for hostlist which is used in idr_torch.py it was hence removed and replaced with our own parser,
• some versions are arbitrary (e.g. torch) and some are even strictly constrained (e.g. mpi4py, flake8 and mypy) these were made flexible.