Missing some external_data items after merge into mainline

[jamesrhester]

The latest version of cif_img.dic in the doc directory is 1.8.5, which (among other things) should contain the external data pointer work that was in jrh_ext_data branch. Comparing the two and correcting for the move to the array_data_external_data category, the following data names are missing and should be added back:

1. _array_data.external_archive_path -> _array_data_external_data.archive_path

This describes the location of a file within an archive. It is distinct from _array_data_external_data.path which describes the location of a frame within a file, e.g. a location within an HDF5 file, which itself might be part of a larger archive.

2. _array_data.external_archive_format -> _array_data_external_data.archive_format

This gives the format of the archive (e.g. .tar.gz) within which the file containing the frame is found.