Revert "SignalSymbolModel: symbols' vector type changed to ISymbol."

This reverts commit f2c1f638a1b68aa78638a0592676c2ee1d693c8f.
ISymbol: added a virtual destructor to make the type to be polymorphic.
Ieee80211OFDMModulator, APSKModulator: followed the changes in the previous reverted commit.