Split up Create banner finction

nmslib.CreateBanner() now calls nmslib.GenRawBanner() for the
majority of processing createbanner now just calls the gen func
and saves the file.

GenRawBanner() returns an image.Image type for more processing by
another function. this will be used for upcoming features.

tests have been updated as needed

upddated go.mod and go.sum

nmslib-glyphbanner_test.go

@@ -10,7 +10,7 @@ func TestCreateBanner(t *testing.T) {
 
 	rp := "21F2F8EDB94D"
 	//rp := RndPortal()
-	fileout := "./test.png"
+	fileout := "./test"
 
 	err := CreateBanner(rp, fileout, 0)
 	if err != nil {