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.
>Why is this function different that NearestWithinSpan (which panics if out of bounds instead of returning NaN)? Because it returns an integer.

`NearestWithinSpan` returns an int to, but yes, I think it should also return -1 for a value that is out of the span now that you mention it.

>Sorry, you are suggesting returning len(s) when v == s[len(s) - 1]? I think supporting the <= on the upper end is useful, for example floats.LogSpan(bounds, minData, maxData) idx = floats.Within(bounds, maxData) You’d otherwise have to add epsilon to the upper range of your data.

This is a philosophical point that has bugged me since as long as I can remember thinking about binning. I actually have no problem with requiring that people put a +Inf on the end of slices to catch things in the bin that has everything else.

>I assume you mean len(s) < 2?

Only if the point immediately above is not accepted.