Add idwtn

This pull request addresses part of #38 (dwt2 has idwt2, but dwtn does not have idwtn). Some smoke tests included but please let me know if there is anything else I should test.

There is also a minor change to bring the variable naming in dwt2 into agreement with the docstring of dwtn - not essential but I feel it makes more sense this way.

(This is the first pull request I've created on github, hopefully this is all OK.)