External group

[scemama]

psi4/psi4#2847 (comment)

We could probably add a functionality allowing to write an arbitrary variable in e.g. "external" group via generic trexio_write|read_(file, variable-str, datatype-str, size-max). I can implement it easily for the HDF5 back end but TEXT one is more tricky.

[scemama]

For the text back-end, we can search for the name of the attribute as usual. If it is difficult, a simple workaround would be to create one file for each external data. This way, the name of the external data would be the name of the text file.

[q-posev]

Yes, one text file per attribute is the easiest way to implement this. I will come back to this when I have more time.