Start exercise 9: weigh values to enhance random selections
Use different parameters for the gauss curves of octaves and durations

While the music is melodious and the major scale is well rendered,
the rhythm still sounds odd: very short notes mixed at random with
longer notes blur any idea of tempo.

I will next try to shift the top of the gauss curve to the first
item for durations, to get a very quick tempo with only very few
longer notes.

I will then add a memory in random_notes, giving a high weight to
durations close to the duration of the previous note, whether it was
fast or slow.
Add missing space after comma in lists
Fix documentation of gauss_weights

The formula in the documentation mentioned the size of the list,
where the actual formula uses separate parameters amplifier/flattener.
Add optional parameter for offset in gauss_weights
Add unit tests for shifted offset

The value used as parameter is the opposite of the value that
makes sense to me: -1 must be provided to place the top of the
gauss curve on the item at index 1.

I will update the definition to make the value compatible with
common sense: +1 to shift the gauss curve to the right, -1 to
shift the gauss curve to the left.
Reverse the sign of offset to put in line with common sense
Add missing space characters after commas
Shift the Gauss curve for durations to the right (center on 1/4)
Do not shift Gauss curve for durations

This option (keeping the Gauss curve centered on 1/8 notes)
is actually the one that sounds the best.