CacheDag: Ability to make methods for desired target outs

[thorwhalen]

The current CacheDag.__call__ is just the backend.
For more convenience, we'd can add methods for all, or selected targeted values.

Consider the CachedDag below.

def add(a, b=1):
    return a + b

def mult(x, y=2):
    return x * y

def subtract(a, b=4):
    return a - b

from meshed import code_to_dag

@code_to_dag(func_src=locals())
def dag(w, ww, www):
    x = mult(w, ww)
    y = add(x, www)
    z = subtract(x, y)

from meshed.scrap.cached_dag import CachedDag

g = CachedDag(dag)
dag.dot_digraph()

We'd like to have methods like g.mult with a signature corresponding to the root nodes it needs to satisfy it's inputs.