Test if a URI is a valid SOMA dataset

[roaga]

There currently isn't a strong way to check if a given URI is a valid SOMA dataset without first initializing a SOMA object. Additionally, exists() (which requires initializing a SOMA object first) only checks if the URI points to a TileDB group, not if it is specifically SOMA data.

[aaronwolen]

Great point and thanks for the suggestion @roaga!

[johnkerl]

As of #133 we have array-level/group-level metadata on SOCOs, SOMAs, and their components …

… however, getting at this currently from a URI requires either (a) instantiating a SOCO/SOMA object, or (b) dropping down to the tiledb (not tiledbsc) API -- this can be improved

[johnkerl]

Resolved by #175