Plot functions

Adds functions to make plots to support existing calculations etc.

Switches existing examples to use these new functions.

Documents and provides examples within plotting function doc-strings too.

Current plots supported:

• Flow duration curve
• Raster hydrograph
• Streamflow duration hydrograph
  • Example using the hyswap function to calculate daily percentiles
  • Example using dataretrieval to fetch daily statistics from the NWIS web service
• Cumulative streamflow hydrograph

PR also fleshes out the documentation a bit and expands some tests to cover more scenarios.

Updated (5/26/2023) to do:

• Add additional plot examples
• Fix the cumulative plotting code
• Check and update labels on plots
• Figure out how to label time-axis with months rather than day of year

Branch built off of !10 (merged) so will one day get merged after that one