Move the ndx output in main

As mentioned in issue #20, the ndx output should not be in the
split_leaflets function. If it includes the ndx output, the
split_leaflets function as two purposes instead of one. Also,
split_leaflets is the closest we have to a entry point if splitleafs is
used as a module. Using the function with the ndx output in it results
in a very long and most likely undesirable print.
Improve the docstring of split_leaflets

The split_leaflets function is the closest to an entry point if
splitleafs is used as a module. Improving its documentation is required
to make splitleafs a decent python module.

See issue #20.
Mention the use as module in the README

The doc introduced here is minimal. A better doc (probably using sphinx)
would be needed.

This commit gives an example of use as a module and point out an entry
point to the module.

See issue #20.
Merge pull request #23 from jbarnoud/as-module

Improve the use of splitleafs as a python module