Search API Solr Dynamic Ranges

This module aims at calculating dynamic ranges from stats delivered by a Solr search database.

The stats would be used to calculate the ranges as follows:
The stats will have a Min, Max, Average and StDev.
The Average tells us something on where most of the results are located. Close to Min, close to Max or close to the middle.
The StDev tells us something about the distribution of the results. A high peak near the Average or a nice even distribution. When there are multiple peaks, we can't tell, but we can still assume a more or less even distribution.

Below is an example of how this would look like. The markers on the x-axis mark the ranges. So there would be six of them in total, with five of them around the Average and the sixth one taking up the upper halve of the complete range.

            -                                       
           / \                                      
          /   -                                     
         -     \                                    
        /       ----                                
      --            \                               
     /               ----------                     
   --                          \                    
  /                             --------------------
  |---|---|--|---|------|--------------------------|
  Min                                            Max

We will make changes to the query sent to the solr search server to request the stats. This will be made available through the workflow on the index. There will be one setting, the field on which to request the stats.

We want to use these ranges in graphs (pie-charts, bar-charts, etc.). Unless we set a hard limit on the number of facets they tend to overcrowd the graphs on (free field) numeric facets. Setting the hard limit is sometimes not desirable. This part will be implemented as a filter on the facet and this is where the heavy lifting is done. Here we have to request the stats for each of the ranges to get the counts on each of them. There will also be one setting on the filter, the (maximum) number of ranges to generate.