[core] Refactoring IOFormats so there is one and only obvious way to send it.

This is the commit where I admit that having more than one input/output
format for readers and writers was complicating the code too much for a
very small gain, and that it would be easier to only have one way to do
it.

So such way is now:

- Returning (or yielding) a dict if you have key-value type collections.
- Returning (or yielding) a tuple if you have a list-type collection.
- Returning (or yielding) something else otherwise, which will continue
  to work like the old "arg0" format.

IOFORMAT options has been removed in favour of a RemovedOption, which
will complain if you're still trying to set it to anything else than the
one value allowed.

tests/test_execution.py

@@ -51,31 +51,31 @@ def test_simple_execution_context():
     graph = Graph()
     graph.add_chain(*chain)
 
-    ctx = GraphExecutionContext(graph)
-    assert len(ctx.nodes) == len(chain)
-    assert not len(ctx.plugins)
+    context = GraphExecutionContext(graph)
+    assert len(context.nodes) == len(chain)
+    assert not len(context.plugins)
 
     for i, node in enumerate(chain):
-        assert ctx[i].wrapped is node
+        assert context[i].wrapped is node
 
-    assert not ctx.alive
-    assert not ctx.started
-    assert not ctx.stopped
+    assert not context.alive
+    assert not context.started
+    assert not context.stopped
 
-    ctx.write(BEGIN, Bag(), END)
+    context.write(BEGIN, Bag(), END)
 
-    assert not ctx.alive
-    assert not ctx.started
-    assert not ctx.stopped
+    assert not context.alive
+    assert not context.started
+    assert not context.stopped
 
-    ctx.start()
+    context.start()
 
-    assert ctx.alive
-    assert ctx.started
-    assert not ctx.stopped
+    assert context.alive
+    assert context.started
+    assert not context.stopped
 
-    ctx.stop()
+    context.stop()
 
-    assert not ctx.alive
-    assert ctx.started
-    assert ctx.stopped
+    assert not context.alive
+    assert context.started
+    assert context.stopped