Unhandled possibility of division by zero in LinearInterpolator.java

Line 339 of LinearInterpolator.java does not take into account the possibility that tmpDifference can be equal to zero, thus returning NaN should this case occur.

Also, the documentation does not really make clear what this method is actually supposed to do. That the method computeT() "returns t" is pretty self evident without the linked comment but what is that value actually supposed to represent?