Review potentially duplicated IGF functions

The table Potential Duplicates lists a number of potential duplicates for the IGF functions:

| IGF_appl_ivas | | IGF_calc_ivas | | IGF_CalculateEnvelope_ivas_fx | | IGF_ErodeSpectrum_ivas_fx | | IGF_getCrest_ivas | | IGF_getSFM_ivas_fx | | IGF_getWhiteSpectralData_ivas | | IGF_gridSetUp_ivas_fx | | IGF_prep_ivas | | IGF_RefineGrid_ivas_fx | | IGF_replaceTCXNoise_2_new_ivas | | IGF_replaceTCXNoise_2_new_ivas_with_var_shift | | IGF_Whitening_ivas_fx | | IGF_WriteEnvelope_ivas | | IGF_WriteFlatteningTrigger_ivas_fx | | IGF_WriteWhiteningLevels_ivas_fx | | IGF_WriteWhiteningTile_ivas_fx | | IGFCommonFuncsIGFConfiguration_ivas_fx | | IGFCommonFuncsMDCTSquareSpec_ivas | | IGFDecApplyMono_ivas | | IGFDecReadData_ivas_fx | | IGFDecSetMode_ivas_fx | | IGFDecUpdateInfo_ivas_fx | | IGFEncApplyMono_ivas_fx | | IGFEncConcatenateBitstream_ivas_fx | | IGFEncSetMode_ivas_fx | | IGFEncWriteBitstream_ivas_fx | | IGFEncWriteConcatenatedBitstream_ivas_fx | | IGFSaveSpectrumForITF_ivas_fx | | IGFSCFEncoderEncode_ivas_fx |

What should be done is

1. Understand the differences between these two variants - how similar or different are they? Does it make sense to harmonize these? Potential reasons keep them separate might be completely different precision (16 vs. 32 bit), too high effort, ...
2. If reasonable, harmonize the EVS and IVAS variant. The floating-point code should be the reference.