Add dataspace creator and NetCDF backend converter engine

[jp-dark]

This is a large pull request that adds the features needed to convert a NetCDF group to a TileDB group. It mostly consists of new code with few changes to the exists core library (moved from tiledb/cf/__init__.py to tiledb/cf/core.py).

Summary:

• Adds external facing classes:

  • ArrayCreator
  • DataspaceCreator
  • DataspaceConverter
  • NetCDF4ConverterEngine

• Adds external facing functions:

  • from_netcdf
  • from_netcdf_group

• Adds additional classes and functions to support new external facing classes and function.

• Updates API docs and testing to support new code.

[normanb]

@jp-dark this looks good, I have installed and run the code and also gone over the changes. A couple of comments;

1. A chore, but can you check the year for the copyright headers (2020 or 2021)?
2. Can you add a CLI interface to the from_netcdf method?