Model code: are looping vs. non-looping sequences properly advanced in time?

[Carsten]

If an animation sequence has four frames 0...3, the maximum frame number maxF is

• for looping sequences: maxF < 4.0,
  because e.g. a value of 3.9 is 10% of frame 3 and 90% of frame 0.
• for non-looping sequences: maxF == 3.0,
  because frame 3 is the last frame in the non-looping sequence.

The subject of this ticket is to make sure that these rules are properly respected and implemented.

[Carsten]

(In [484]) Model code:

• Moved the "force loop" feature from being a parameter to AnimExpressionT::AdvanceTime() to a property of AnimExprStandardT.
• Clarified how looping vs. non-looping sequences are correctly handled, and updated related methods AnimExprStandardT::AdvanceTime() and AnimExprStandardT::GetData() accordingly.

Closes #99.