Compute minimum peak

1. apply pseudo code
2. create compute_map and node_mem in new method
You don't really need to copy the compute_map here.  You can generate a new one in the count_minimum_peak() function.  It's just a dictionary with the variables of the graph as keys and booleans as values.

If you don't feel like going through the graph to find all the variables, just use a [defaultdict()](https://docs.python.org/2/library/collections.html#collections.defaultdict) that defaults to False.