Battery checking should be continuous, not just at steps

[Mark2000]

Describe the bug
The satellite can die then come back alive over the course of a step without triggering failure.

Additional context
Should be fixed by either logging battery and checking at the end of the step, or by adding a continuous terminal event checking battery level. I think the latter is preferable, as it will be less memory intensive (or maybe not, turns out events are time-expensive to check.)

[Mark2000]

Relevant to #74 , once the battery reaches 0 the satellite should remain dead.