Add option to specify compression type of individual files when accessed externally

[jamesrhester]

As per issue #37 , individually-compressed files are not covered by the current specification. This pull request adds a new item, _array_data.external_file_compression to fix this.

I've also adjusted the wording elsewhere to specify that accessing SMV files returns the array, not a byte stream.

[yayahjb]

I believe this is now covered by the new archive tags. I am going to close with
this comment, but feel free to reopen if still needed -- Herbert

[jamesrhester]

This is a separate tag that is not covered by the previous ones. This tag covers the situation in which the individual files referred to by the URI or contained in the archive are themselves compressed. It seems to be common with the proteindiffraction.org archive, for example, so this should remain open. It is not possible for me to reopen this.