Turn arias duration utility into a module, added rod velocity module

[kevinmilner]

This pull request includes a couple old un-merged changes that I made in 2020. Can you take a look and let me know if you think any will mess with other parts of your workflows that I don't use?

New RotD velocity module

This module computes RotD velocities, useful for computing PGV. It uses the same RotD100 fortran code, but I also modified that code to include 2 additional (short) periods: 0.0001, 0.001

I also modified bbp_formatter.py such that the bbp2peer method can write out velocities if the new optional argument accel=False (it defaults to true)

Arias duration utility is now a module

There was an existing arias duration utility class (arias_duration.py), I turned it into a module and modified it to also spit out 20/80 durations.