Add activities to queue

# Feature
## Desired Behavior / Functionality

A user should not have to keep track of when it is allowed to submit activities to PASEOS. Rather, the user may submit activities to a queue of activities that gets performed on a first-in first-out basis by PASEOS.

## What Needs to Be Done
Implement a function queue_activity() and have PASEOS check the queue once an activity is finalized.

## How Can It Be Tested
Make a test that submits multiple activities via the queue and check that each gets executed properly.