fix bugs in start_flowgraph.py

[reiinakano]

We're replacing PyYaml with ruamel.yaml which is a maintained fork of PyYaml. https://yaml.readthedocs.io/en/latest/pyyaml.html

We ran into this bug https://stackoverflow.com/questions/30458977/yaml-loads-5e-6-as-string-and-not-a-number