Automatic to_<tag> Python method

[msoeken]

In the Python interface a write_<tag> command can be used to write the file contents to a Python variable using, e.g.,:

f = cli.write_html(log=True)["contents"]

This PR automatically adds to_<tag> methods to the Python interface for each write_<tag> method, making the code simpler, e.g.:

f = cli.to_html()

Make sure to implement the write method from the store API that writes into an output stream.