Major revision to tree.py

[eugenwintersberger]

Original reporter: rayosborn

As I discussed on the NeXus mailing list a few weeks ago, there has been a major rewrite of the tree API that makes the API more robust and more convenient to use interactively. The major changes are:

1. to give the NXfields (formerly SDSs) all the functionality of Numpy arrays

2. to allow group items to be accessed through a dictionary interface

3. to make the syntax cleaner by simplifying method names.

I will be uploading a new version shortly, which will not be backwardly compatible with the previous one. Please contact Ray Osborn ROsborn@anl.gov if you need assistance upgrading to the new version.

[eugenwintersberger]

This issue was originally nexusformat/code#322.