Fix: clarify API

[chrisdicaprio]

The current names for properties and methods is ambigouous since rate can mean either a slip rate or a rupture rate.

• [x] all methods and properties with rate in the name changed to slip_rate or rupture_rate
• [x] column headers in dataframes specify if slip rates are constraints (input) or solution (output)
• [ ] column headers in dataframes specify rates to be slip or rupture - PARKED to avoid conflict with opensha file format