Conversions related to IGFEncApplyStereo added

[x] Cleaned up IGFEncApplyStereo_fx
[x] Converted IGF_Whitening_ivas_fx

EVS 26.444 BE test passes.
decoder pytests:
MLD scores:
Original inputs: 562 failed, 374 skipped
MLD > 10: 37
MLD > 5 : 94
MLD < 5 : 431
+10dB inputs: 562 failed, 374 skipped
MLD > 10: 149
MLD > 5 : 187
MLD < 5 : 226
-10dB inputs: 562 failed, 374 skipped
MLD > 10: 8
MLD > 5 : 44
MLD < 5 : 510

encoder pytests results:
MLD scores:
Original inputs: 562 tests:
MLD > 10: 15
MLD > 5 : 47
MLD < 5 : 500
+10dB inputs: 562 tests:
MLD > 10: 45
MLD > 5 : 130
MLD < 5 : 387
\-10dB inputs: 562 tests:
MLD > 10: 5
MLD > 5 : 21
MLD < 5 : 536