Add a demo for `baseflow.fitab`

[mgcooper]

Is your feature request related to a problem? Please describe.
Simple usage to estimate parameters a and b using baseflow.fitab is not currently demonstrated by any demos.

Describe the solution you'd like
Add a demo showing how to use baseflow.fitab to estimate these parameters from a single or just a few recession timeseries.

Describe alternatives you've considered
n/a

Additional context
n/a