add POSCAR.lobster support in featurizer

[naik-aakash]

Changes

1. Checks if POSCAR.lobster exists instead of POSCAR then use it as default structure file

[naik-aakash]

Hi @JaGeo, I think this is all that is needed.

In CLI, already we have the option to switch structure files, so do we such manupulation there as well or I mention this thing in the documentation and it should be enough? (I feel mentioning it in documentation is enough, but wanted your opinion on this)

[JaGeo]

Just mention it in the documentation. Thanks!