Model and document sensor and electronics-chain readout non-linearities

[cwwalter]

Although quite small (based on test-stand measurements) we should eventually add the ability to put in electronics non-linearity.

As of 09/19/2018 information from @jchiang87 is that:

"The most recent raft tested at IR2 has linearity < 0.2%. The acceptance spec < 3%"

This issue also serves to document our current understanding. This information should be propagated to the imSim feature matrix on the Wiki.