got rid of obsolete methods for IsSingularPoly
renamed RingMacrosForGAPWithSingularForHomalg init_string

$init_string_SingularForHomalg -> !init_string_SingularForHomalg

(needs HomalgToCAS v2013.01.09:
 InitializeMacros now supports '!'-entries for init_strings)
renamed our SI_-functions -> SIH_* and outsourced them to SIH_Functions.g?