Improve consistency of terminology in documentation

[NickEntin]

We should be more consistent with the terminology we use in the project - both in the long form documentation and headerdocs, as well as some of the (private) variable names. For example, when talking about durations, the "duration" can apply to different parts of the animation, so I've started differentiating by saying:

• The time from the start of the execution phase to when the animation completes is the end-to-end duration. This is usually a calculated value based on the cycle duration and the repeat style (when available).
• The time for the animation to go from a relative timestamp of 0 to 1 (or 1 to 0) is the cycle duration. This is typically how consumers specify the duration.
• The time for the animation to go between two arbitrary relative timestamps is the segment duration. This is used in a few places like snapshot testing and the upcoming interactive animations.

See original suggestion in #49 (comment)

[NickEntin]

And @seanho's response to that comment:

This makes sense, total can be ambiguous when animation has multiple phases. end-to-end sounds good but feels a bit clumsy as variable name, what about overall duration?