Improve TDSCatalog walking

[dopplershift]

Some ideas for improving walking through the catalog:

• Implement walk() that would allow blowing through the nest hierarchy, e.g. cat.walk('Channel02/current')
• Another option is to follow pathlib with something like: cat / 'Channel02' / 'current'
• Implementing either/both, we need to use the hooks for IPython that allow for tab completion. I'm not sure if it will work for the above options, or if that's only for attribute/dictionary access; in the latter case we should instead go for an API that allows for it, since we really want to ease quick, notebook-based exploration
• A lot of this will also be improved with better string representation of the objects, as mentioned in #260