cg-proc: Option to treat '~' as compound separator

[unhammer]

Cf. apertium/lttoolbox#177 it should be possible to treat ~ as a subreading separator when reading/writing with cg-proc

[TinoDidriksen]

Sounds like a trivial modification to https://github.com/GrammarSoft/cg3/blob/main/src/ApertiumApplicator.cpp#L597