add a plot showing the network flow at a point

[zingale]

This reads the data from the location of peak enuc and uses pynucastro to visualize
the network flow

PR summary

PR motivation

PR checklist

• test suite needs to be run on this PR
• this PR will change answers in the test suite to more than roundoff level
• all newly-added functions have docstrings as per the coding conventions
• the CHANGES file has been updated, if appropriate
• if appropriate, this change is described in the docs

[zingale]

here's an example

[zingale]

this is ready for review now.

[zingale]

here's the current output -- it uses the net rate functionality recently merged into pynucastro: