Animate specification text improvements.

[skynavga]

A number of improvements are needed to clarify both syntactic constraints and semantics of the animate element:

• improve description of constraints/clarifications information
• specify implied defaults for keyTimes, keySplines, repeatCount
• specify constraints on keyTimes and keySplines
• specify default semantics for additive and accumulate semantics (in the absence of these attributes)
• miscellaneous stylistic improvements

[skynavga]

Questions to consider:

1. at present, we don't support additive or accumulate; should we add support now (or defer to TTML3, etc)?
2. in SVG, repeatCount can be a non-integer, allowing repetition of a portion of the simple duration; should we add support now or stick with integral repetition as defined?

[nigelmegitt]

Thanks @skynavga are those all the questions that need addressing? If there are other parts of this issue that need to be resolved please describe the problems here in the issue; then we can assess the pull request against the issue to see if it resolves the problems.