Improved handling of how to call Python 3

vgen_process_simulink_model.m

@@ -25,10 +25,16 @@
 % this is where hdlworkflow puts the vhdl files
 hdlpath = [mp.modelPath filesep 'hdlsrc' filesep mp.modelAbbreviation];
 
-if ispc
-    python = "python ";
-else 
+if system("python3 --version") == 0
     python = "python3 ";
+else 
+    [status, output] = system("python --version");
+    if status == 0 && contains(output, "Python 3.")
+        python = "python ";
+    else 
+        disp('Python 3 was not found on the path. This must be fixed before continuing')
+        return;
+    end
 end
 
 %% Generate the Avalon VHDL wrapper for the VHDL code generated by the HDL Coder