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 improve repr of siphon objects #260

[kthyng]

@dopplershift Hi! I'm trying out siphon for thredds handling and getting opendap links out. Trying with a few different thredds servers, I have needed to set up different code to dig down to what I need and I think it is dependent on whether the thredds catalog is nested or not, which brought me to this issue. Is there a way to handling nested catalogs with siphon? If not do you know of another package that would? I saw intake-thredds which would be great but it doesn't look updated to intake v2.

[dopplershift]

Can you provide a link, or preferably sample code, that's not opening a catalog in a way that you expect? It would be easier to give you tips to help you on your way if we're looking at the same thing.