कथयति / कथापयति - optional forms according to शाकटायनः

[drdhaval2785]

https://github.com/funderburkjim/elispsanskrit/issues/43#issuecomment-247569133

This gives an explanation of the optional form of kaTApayati etc. @funderburkjim I am not going to implement this in SanskritVerb as of now. It is too drastic. And there is no Paninian rule which mandates it. Panini has applied this 'Apa' to a finite number of verbs, unlike zAkaTAyana who thinks that this applies to all 'a'-ending verbs of 10th class.

Noting it here, in case in future I decide to include them.