Initial group/array metadata access methods

[johnkerl]

This introduces some metadata accessors but doesn't yet put them to use.

Places to use them include #106, where:

• For columns, we'll store original category-type names (for anndata categoricals which were downcast to TileDB strings)
• For arrays, we'll store information like "this was originally CSR" or "this was originally CSC"
• For uns, we'll store scalars no longer as length-1 arrays but rather as group metadata
• For SOMA objects, we'll store information like "this was written by tiledbsc-r v0.1.2

See also #115 which is stacked atop this.

Note: this is stacked atop #144, hence the higher line-count -- see https://github.com/single-cell-data/TileDB-SingleCell/pull/116/commits

[aaronwolen]

Nice!