Added Within function.

Within is useful for finding the bin of a datapoint, for example when making a histogram or otherwise trying to find similar points. This commit adds the function and tests.
I also have no problem with requiring people to put a +/- inf to catch everything else. I agree that's the better way to do it. However, it is a common case that you have data that falls between min and max, and you want to bin it. Within should work when the last element of s is max, and when the value is max, i.e. Within should be [min,max] and not [min,max). I don't want users to have to use math.NextAfter() to avoid the maximum value returning -1.