Refactor automatic translation of LDLf in LTLf in LTLfParser

[marfvr]

The current behaviour of LTLfParser is to directly translate LTLf formulae into LDLf formulae.

This is not ideal in general, especially for programmatic usage. It conflates two concepts, parsing and translation, into one class. Would be better to have a separate visitor class that does the translation from LTLf to LDLf.