Refactor and improve visual inspection tools

[mzargham]

The inspect_subgraph.py file contains an early version of a quick plotting utility for sourceCred type graphs. In addition to refactoring the existing inspect subgraph method, it is necessary to transfer a handful of common plotting routines from notebooks into pre-infra files, including but not limited to improving axis labels and parameterizing methods for broader use.

Existing file is here:
https://github.com/sourcecred/research/blob/mz-exploratory/exploratory/mZargham/pre-infra/inspect_subgraph.py

samples of plotting methods can be found in this notebook:
https://github.com/sourcecred/research/blob/mz-exploratory/exploratory/mZargham/pre-infra/circle.ipynb