Replace duplicated file by a symlink

[susilehtola]

Closes #544

[ilfreddy]

I believe this has to stay the way it is.
The regeneration of the input parser requires a package that we don't require for the MRChem installation to avoid too many conflicts.
If we replace the file with a symlink it will be eventually overwritten the next time the input parser is regenerated
If we remove the file altogether (instead of the symlink) we will have to require the additional package (parselglossy I guess...) for regular installations.

Conclusion: the file duplication here is possibly less elegant but more practical.

[ilfreddy]

I believe this is connected to the (closed) #507

[susilehtola]

Yet, both files have the disclaimer that they are autogenerated.