Set a reader library instead of file suffix

[PicoCentauri]

It might make sense to invert the control flow here. Instead of guessing the reader from the file extension, do something like

structures:
    read_from: file.xyz
    reader: ase

or

structures:
    read_from: ase://file.xyz

this way we don't try to use ASE on CP2K xyz files, which are not quite compatible

Originally posted by @Luthaf in #84 (comment)

I would go for the first option and leave the reader as a optional field. If the user does not specify a reader library we will infer it from the file suffix based on a list of known extensions (as we are currently doing it).

[Luthaf]

The main advantages I see for the second option is that which reader is being used is made explicit, so any error would be less surprising to an user. But it is a bit more clunky than just trying to guess the best option for a file from it's extension.

I'd be fine with either solutions here!

[DavideTisi]

I like the first option more, it seems more direct

[PicoCentauri]

We can try to formulate a good error message like

"`dataset.foobar` could not be parsed succesfully by the reader library `ase`. "
"Try to change `reader` field in your `options.yaml`. "
"Possible reader libraries are ase, deepmp, ...."

or similar.