program unit tests for Orbit functions

[mdbernard]

For each function except the Event Sequence controlling functions used across all this team's scripts, write unit tests that check for the following:

• outputs are as expected for nominal inputs
• function is prevented from crashing due to common software errors (e.g. division by zero) (output a "safe" value rather than allowing the system to crash)

Create a checklists of functions in this issue so that progress can be tracked.