renamed entries_from_path to entries_from_dir
adjusted test function names to latest change
save a list of entries per file

- dropped from_fits_filepath and add_fits_header_entries_from_file
- added new utility function entries_from_file
updated database.tables.__all__
fixed example of DatabaseEntry.from_query_result_block
make sure that the `physobs` attribute is converted to str
fixed example of entries_from_query_result
DatabaseEntry._from_query_result_block is no longer part of the public API
make entries_from_dir use entries_from_file