Bring dwt2 into agreement with dwtn docstring

The behaviour and variable naming of dwt2 did not match what was
described in the docstring of dwtn. This brings the two into agreement
(renames LH/HL and corrects V/H respective to 'ad' naming scheme).
Add inverse n-dimesional dwt.

Adds inverse n-dimensional dwt. Smoke tests to ensure it behaves as
(i)dwt2 for 2D case and can reconstruct data. Note resulting dimensions
will always be even, I cannot see any other way of doing it.