Cleanup validation for declared field formats

[roskakori]

Goal: parameters for fields.AbstractFieldFormat.__init__() are validated by there instead of mostly in interface.Cid.add_field_format().

The necessary changes are basically:

1. Add optional location to fields.AbstractFieldFormat.__init__(); if None, set it to errors.create_caller_location().
2. Move validation code for field declarations from interface.Cid.add_field_format() to fields.AbstractFieldFormat.__init__()
3. Simplify reading of a field row by appending empty optional rows instead of repeated if item_count >= ... queries.