Add details for mmCIF external files.

Populate the newly-created _ihm_external_files.details field
with human-readable information about each externally-linked
file. Relates ihmwg/IHMCIF#22.

pyext/src/mmcif.py

@@ -648,11 +648,12 @@ def map_repo(r):
 
     def dump_locations(self, writer):
         with writer.loop("_ihm_external_files",
-                         ["id", "reference_id", "file_path"]) as l:
+                         ["id", "reference_id", "file_path", "details"]) as l:
             for loc in self._location_by_id:
                 repo = loc.repo or self._local_files
                 l.write(id=loc.id, reference_id=repo.id,
-                        file_path=repo._get_full_path(loc.path))
+                        file_path=repo._get_full_path(loc.path),
+                        details=loc.details or _CifWriter.omitted)
 
 
 class _DatasetDumper(_Dumper):

test/test_mmcif.py

@@ -505,7 +505,8 @@ def flush(self):
         l = IMP.pmi.metadata.FileLocation(repo=repo2, path='baz')
         dump.add(l)
         # Repository containing a single file (not an archive)
-        l = IMP.pmi.metadata.FileLocation(repo=repo3, path='foo.spd')
+        l = IMP.pmi.metadata.FileLocation(repo=repo3, path='foo.spd',
+                                          details='EM micrographs')
         dump.add(l)
         with IMP.test.temporary_directory() as tmpdir:
             bar = os.path.join(tmpdir, 'bar')
@@ -541,11 +542,12 @@ def flush(self):
 _ihm_external_files.id
 _ihm_external_files.reference_id
 _ihm_external_files.file_path
-1 1 bar
-2 1 baz
-3 2 foo/bar/baz
-4 3 foo.spd
-5 4 %s
+_ihm_external_files.details
+1 1 bar .
+2 1 baz .
+3 2 foo/bar/baz .
+4 3 foo.spd 'EM micrographs'
+5 4 %s .
 #
 """ % absbar)