Add metadata method to NCSS

[coastwatch]

It would be ideal to add a method to metadata objects created by Siphon, such as "ncss.metadata".
Something like metadata.inventory(), or something to that effect, that would list the properties of interest.

Thanks!

[dopplershift]

I think this is referring to catalog-level metadata. Currently, all catalog metadata are added as properties as they are found during parsing. Instead:

1. Don't pollute the Dataset object with extra properties
2. Hide all of them in a metadata (or something) dictionary, which would improve discoverability