Sort out proper usage of "command signal" versus "reference signal"

[Richard Murray]

In Ch 2 we use the term "command signal" for the input to a control system and talk about the reference value $r$ as an example. In other places we use the term "reference signal". Need to decide on a word pattern and be consistent.

[Richard Murray]

In the index for the first edition I found the following comment:

% command signal for general inputs
% reference signal in tracking applications
% avoid use of reference value
% setpoint in PID (for roughly constant r)

Need to go through and see if this allows for a consistent usage in Ch 2 versus other chapters.

[Richard Murray]

Issue #19 was marked as a duplicate of this issue.

[Richard Murray]

Changed command signal following to reference signal tracking in most places (including command -> reference and following -> tracking). Also cleaned up "tracking mode" indexing. Updates in commit 70995b5.