Add DAG.sequential

[lchu-ibm]

Sequential workflow is common, and it is helpful to add a convenient api to make it simple.

For example, for a DAG of a -> b -> c -> d -> e

Currently we have:
dag.add_edge(a, b, 0)
dag.add_edge(b, c, 0)
dag.add_edge(c, d, 0)
dag.add_edge(d, e, 0)

new api would be:
dag.sequential([a,b,c,d,e])

To do so, items to be done:

1. make default value 0 to add_edge's in_arg_mapping parameter
2. create sequential api which under the hood invoke multiple add_edge

[lchu-ibm]

6b8fd04