Added flows module

This new module introduces two new functions which help in the creation
of dataflows.

The first function is `req_reply()`, which creates two dataflows.
The first is usual dataflow and the second is a dataflow which is a
response to the first.
I used this function a lot for modeling dataflow with databases.

The second functions `reply()` takes a dataflow as an argument and returns
a new dataflow which is a response to the given dataflow.
This function is quite useful for modeling proxies.

```python
d1 = Dataflow(client, web, 'Get page')
d2,d3 = req_reply(web, db, 'Query data')
d4 = reply(d1, 'return web page')
```

pytm/flows.py

@@ -0,0 +1,56 @@
+from pytm import Dataflow as DF
+from pytm import Element
+
+
+def req_reply(src: Element, dest: Element, req_name: str, reply_name=None) -> (DF, DF):
+    '''
+    This function creates two datflows where one dataflow is a request
+    and the second dataflow is the corresponding reply to the newly created request.
+
+    Args:
+        req_name: name of the request dataflow
+        reply_name: name of the reply datadlow
+                    if not set the name will be "Reply to <name>"
+
+    Usage:
+        query_titles, reply_titles = req_reply(api, database, 'Query book titles')
+
+        view_authors, reply_authors = req_reply(api, database,
+                                                req_name='Query authors view',
+                                                reply_name='Authors, with top titles')
+
+    Returns:
+        a tuple of two dataflows, where the first is the request and the second is the reply.
+
+    '''
+    if not reply_name:
+        reply_name = f'Reply to {req_name}'
+    req = DF(src, dest, req_name)
+    reply = DF(dest, src, name=reply_name)
+    reply.responseTo = req
+    return req, reply
+
+
+def reply(req: DF, **kwargs) -> DF:
+    '''
+    This function takes a dataflow as an argument and returns a new dataflow, which is a response to the given dataflow.
+
+    Args:
+        req: a dataflow for which a reply should be generated
+        kwargs: key word arguments for the newly created reply
+    Usage:
+        client_query = Dataflow(client, api, "Get authors page")
+        api_query = Dataflow(api, database, 'Get authors')
+        api_reply = reply(api_query)
+        client_reply = reply(client_query)
+    Returns:
+        a Dataflow which is a reply to the given datadlow req
+    '''
+    if 'name' not in kwargs:
+        name = f'Reply to {req.name}'
+    else:
+        name = kwargs['name']
+        del kwargs['name']
+    reply = DF(req.sink, req.source, name, **kwargs)
+    reply.responseTo = req
+    return req, reply