String Representaion of Fields uses instance ID instead of values

[axlan]

File "/home/axlan/virt_envs/gnss_analysis/local/lib/python2.7/site-packages/okcompute/okc.py", line 615, in run
    skipped_metrics = graph.prune_outputs(desired_output_fields)
  File "/home/axlan/virt_envs/gnss_analysis/local/lib/python2.7/site-packages/okcompute/okc.py", line 224, in prune_outputs
    for d in self.G.predecessors(node):
  File "/home/axlan/virt_envs/gnss_analysis/local/lib/python2.7/site-packages/networkx/classes/digraph.py", line 793, in predecessors
    raise NetworkXError("The node %s is not in the digraph." % (n,))

This may be specific to python2 since most testing was on python3. Probably just need to update some of the magic functions. https://rszalski.github.io/magicmethods/

[axlan]

Was actual caused by removing NamedTuple parent from Field and MetricSpec.

[axlan]

Fixed in #4