Re-implementing gain setter for filter banks

include/ITAThirdOctaveFilterbank.h

include/ITAThirdOctaveFilterbankFIR.h

include/ITAThirdOctaveFilterbankIIR.h

src/ITABiquad.cpp

src/ITAThirdOctaveFIRFilterGenerator.cpp

src/ITAThirdOctaveFilterbankIIR.cpp

tests/ITADSPThirdOctaveFilterGeneratorTest.cpp

tests/ITADSPThirdOctaveFilterbankTest.cpp