Change way readers are chosen based on a file suffix

[PicoCentauri]

          Not a comment for this PR, but long term 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)

[PicoCentauri]

Close in favor of #153