Feature: Astropy/FITS table validation

This adds a new metadata item: fits_column_dtype to AstroField, which allows that field to be used to validate an astropy Table column.

It also adds two new conversion functions:

• model_to_astropy_table, which creates an empty Table object from a model
• model_validate_astropy_table which validates an existing Table object columns
• validate_fits_bintable_hdu helper to validate both a BINTABLE schema and it's metadata in one step.

This allows a user to define a model class for the column schema, and use existing functionality to validate the metadata.