Improve Ricerca handling of negative examples

[spli]

The handling of negative results in Ricerca doesn't take into account the dissimilarity score, only the ranking:
​https://github.com/icaoberg/ricerca/blob/17e0252b49d1f197553cd6d1319e143631ebf9f3/ricerca/content.py#L151

This means when the negative set of samples is very spread out, and for many positive and negative samples d([-],x) >> d([+],x), a negative image can still come out ahead of what you'd intuitively expect to be a positive sample- see the following 1D example below where x1- is ranked above x2+ and x3+. This is probably an even bigger problem with high-dimensional data.

[-] = negative reference
[+] = positive reference
x1, x4: Negative samples
x2, x3: Positive samples


0        9   13 16           29                        55

-        +  [+] +           [-]                        -
|        |      |                                      |
x1       x2     x3                                     x4

                              rev.   avg
    d[+]  d[-]  rank+  rank-  rank-  rank
x1   13    29    3      4      1     2
x2    4    20    2      2      3     2.5
x3    3    13    1      1      4     2.5
x4   42    26    4      3      2     3