refactor code so that it can be used to parse standalone lines

[bernstei]

The original fortran libAtoms extxyz comment line parser was the same as the fortran command line parser. It would be nice to restore this compatibility by making the new extxyz comment line parser usable for the command line passed to a (fortran, but anything, really) program. This requires refactoring the new extxyz code so that the part that parses the comment line can be called with a string. independently of the rest of it, just returning a dict like info.

[jameskermode]

This would also be needed to allow this library to be used by OVITO, see https://matsci.org/t/cartesian-vs-scaled-coordinates-reading-extxyz-files/51746