[chem]: invite for a code fix

Dear Hans,
Dear knowledgeable in groff/chem .and. Perl,

earlier I filed bug to https://savannah.gnu.org/bugs/?68127 which would require to substitute present

$Params{'db'} = $scale * 0.2;

in file groff/contrib/chem/chem.pl (line 1259) about bond lengths outside the cyclic templates (ring3, ring4).  Preferably, it would  to be conditional, i.e.

- either scaled by a factor of 0.3 like the side of the rings for a plain bond (as defined by e.g., `bond 120`), or
- scaled by 0.2 with an explicit atom / atom group defined (as in `bond 120 ; OH`)

Since I lack the necessary experience with Perl, I thought one of you might be interested to prepare and eventually file PR.

Regards,
Norwid