Harmonize ReadTnsData_ivas_fx()/ReadTnsData_fx() and DecodeTnsData_ivas_fx()/DecodeTnsData()

Bug description

The function pairs

• ReadTnsData_ivas_fx() / ReadTnsData_fx()
• DecodeTnsData_ivas_fx()/DecodeTnsData()

seem to have potential for harmonization. It should be checked whether

• ReadTnsData_ivas_fx() / DecodeTnsData_ivas_fx()

  corresponds to the floating-point code and

• whether ReadTnsData_ivas_fx() / DecodeTnsData_ivas_fx()

  could replace ReadTnsData_fx() (after being renamed to ReadTnsData_fx() ) and DecodeTnsData() (after being renamed to DecodeTnsData_fx())