Field3d - add helpful metadata

When reading Field3d files, add metadata in the image spec giving the raw FieldRes pointer (use with extreme caution!), the field data type, and the complete lists of SparseField blocks.

Also some minor changes to ImageSpec::format_raw_metadata that allows more graceful printing of pointers and long arrays.  (Helpful when doing something like "iinfo -v foo.f3d" now that this metadata is present.)