asic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">here
Preconditions:
- The observed array length must be at least 2.
@param mu constant value to compare sample mean against
@param sample array of sample data values
@return p-value
@throws IllegalArgumentException if the precondition is not met
@throws MathException if an error occurs computing the p-value