ENH: Support extra metadata

[DPeterK]

All of a NetCDF file's metadata is stored in the in-memory representation of the NetCDF file provided by NCToTDB, but not all of it is passed through to the writer when a TileDB array is created. Notably this includes auxiliary and scalar coordinates. These should be added to the TileDB representation of the NetCDF file.

Here's a list of metadata that should be supported:

• scalar coordinates
• aux coordinates
• grid mapping (Pass grid mapping metadata #24)
• bounds

This list may not be exhaustive.

[DPeterK]

Grid mapping done in #24.