Add NaN Safe Assert Statements

[kylekrol]

Currently, some assert statements in GNC will fail on NaN inputs even when a NaN input is permissible. Adding some assert macro(s) that allows a NaN inputs would be helpful.

[kylekrol]

This bug was moved into the critical category because it recently hindered our ability to run the attitude filter during eclipse in PSim standalone. Essentially, when the satellite goes into eclipse the sun sensor measurement is set to NaN. This causes assertions to fail in the gnc library.

Two things to note:

1. The assertions should be fixed so we can run psim standalone with assertions present.
2. We should also be able to build a "release" version of the Python module that excludes assertions for the sake of a performance boost. This, however, seems better tracked on Add a Mechanism for Debug and Release Builds #243.