Corrected minor bug in GBneck screening parameter assignment. The existing logic
was:

if symbl[0] == ('c' or 'C'):
   ...do stuff...

This logic will match only 'c' as ('c' or 'C') evaluates to 'c' while
('c' and 'C') evaluates to 'C':

>>> 'c' or 'C'
'c'
>>> 'C' == ('c' or 'C')
False

The correct logic is either

if symbl[0].upper() == 'C':
   ...do stuff...

or

if symbl[0] in ('C', 'c'):
   ...do stuff...

(Alternatives can be [ if symbl[0] in 'cC' ] or [ if symbl[0].lower() == 'c'])
Add a comment to clarify that calling setReactionFieldDielectric on force
instead of gb is intentional.