Add missing functionality in the Python wrapper.

Tweak the Python version of zgydump to allow it to be run both as a script and a module. Also make it even more tolerant of unsupported attributes. The idea is that zgydump.py can now be run both with the pure Python API and the wrapper API. Which will flag some of the missing attributes.

Add indexcorners, annotcorners, brickcount, and filestats to the wrapper API. These were not present in the old Python API but they exist in C++ and it makes sense to support then in the wrapper as well.