Add functions to main scripts to be entry_points

Add functions to ck2cti, ctml_writer, and a new file __main__.py to
mixmaster to be the entry_points locations that setuptools scripts
will call.

interfaces/cython/cantera/ck2cti.py

@@ -2028,5 +2028,8 @@ def main(argv):
         print(e)
         sys.exit(1)
 
+def script_entry_point():
+    main(sys.argv[1:])
+
 if __name__ == '__main__':
     main(sys.argv[1:])

interfaces/cython/cantera/ctml_writer.py

@@ -2681,8 +2681,11 @@ def convert(filename=None, outName=None, text=None):
 
     write(outName)
 
-if __name__ == "__main__":
-    import sys
+
+def main():
     if len(sys.argv) not in (2,3):
         raise ValueError('Incorrect number of command line arguments.')
     convert(*sys.argv[1:])
+
+if __name__ == "__main__":
+    main()

interfaces/cython/cantera/mixmaster/__main__.py

@@ -0,0 +1,8 @@
+from cantera.mixmaster.main import MixMaster
+
+
+def main():
+    MixMaster()
+
+if __name__ == '__main__':
+    main()