[algorithm] Add loop information parsing

The format is:

"loop": {
    "input": {
        "type": "data_format_name"
    },
    "output": {
        "type": "data_format_name"
    }
}

beat/backend/python/algorithm.py

@@ -419,6 +419,8 @@ class Algorithm(object):
                 for k,v in g['inputs'].items()])
         self.output_map = dict([(k,v['type']) for g in self.groups \
                 for k,v in g.get('outputs', {}).items()])
+        self.loop_map = dict([(k,v['type']) for g in self.groups \
+                for k,v in g.get('loop', {}).items()])
 
         self._load_dataformats(dataformat_cache)
         self._convert_parameter_types()
@@ -457,6 +459,23 @@ class Algorithm(object):
 
                 self.dataformats[output['type']] = thisformat
 
+            if 'loop' not in group:
+                continue
+
+            for name, entry in group['loop'].items():
+                entry_format = entry['type']
+                if entry_format in self.dataformats:
+                    continue
+
+                if dataformat_cache and entry_format in dataformat_cache:
+                    thisformat = dataformat_cache[entry_format]
+                else:
+                    thisformat = dataformat.DataFormat(self.prefix, entry_format)
+                    if dataformat_cache is not None:
+                        dataformat_cache[entry_format] = thisformat
+
+                self.dataformats[entry_format] = thisformat
+
 
         if self.results: