ENH: Update PolygonClassStatistics, SampleSelection and SampleExtraction applications so as to read possible field values and expose them as a ListView parameter

ENH: Update TrainVectorClassifier application so as to read possible cfield values and expose them as a ListView parameter. Also filter feat field on type (avoid string fields)

PKG: use only otbenv.profile. side effect is that it set PATH but that won't be affected otbcli and otbgui scripts