Allow nnz=1 in the destriper

[ziotom78]

This small PR lets the destriper run if one specifies nnz=1 (i.e., only solve for intensity I).

[ziotom78]

I had to avoid using poetry in CI builds because of this bug, so the YAML recipe exports a plain requirements.txt file and uses it to set up the environment.

[ziotom78]

Now the part modifying the dependencies has been incorporated in #192, so that the history for this PR will be cleaner when merged.