plot_network: check nan values of coh/bperp in kept ifgrams

[yunjunz]

Description of proposed changes

• plot_network: raise error for pairs with invalid coh/Bperp value, probably due to upstream insar processing issues. This is rarely seen, as reported in the google group.

• utils.plot.auto_adjust_yaxis: ignore nan value in input data

  • use np.nanmin/nanmax() to replace min/max() to ignore nan values in the input data list
  • light refactoring to improve the readability

• utils.plot.py: use consistent parameters/returns comment styles for all functions

Reminders

• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
• Make sure that your code follows our style. Use the other functions/files as a basis.
• If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.