New layer-stats tool to explore data

[nyurik]

The tool is modeled on the OpenMapTiles qa/ tools. The tool shows some statistics for a given layer's field for each zoom. It currently supports 3 methods:

• frequency -- Shows how often each unique value occurs in a layer's column. If more than one column is given, shows unique combinations.
• toplength -- Shows the longest N values for a given layer's column.
• variance -- Shows a few statistical metrics for a column's numeric value.

This is a simple test script I used outside of the docker, and it could be adapted to run inside the docker as well. https://gist.github.com/nyurik/457102422adfb380ec9e8552d7656112

@TomPohys we can merge this and then add a new flag, e.g. --markdown to generate .md output.

Sample output:

$ layer-stats frequency openmaptiles.yml landcover class subclass
======= Analyzing value occurrence in layer 'landcover' for fields [class, subclass] =======
class    subclass      z0    z1    z2    z3    z4    z5    z6    z13    z14
-------  ----------  ----  ----  ----  ----  ----  ----  ----  -----  -----
ice      glacier       11    11   377   377   377  1886  1886
ice      ice_shelf                 64    64    64   159   159
grass    garden                                                    2     29
grass    park                                                      2     12
grass    grass                                                            1
sand     beach                                                            4
wood     forest                                                           2
wood     wood                                                             3


$ layer-stats toplength openmaptiles.yml water_name name_en
======= Analyzing longest field values in layer 'water_name' for field [name_en] =======
name_en                       z14
--------------------------  -----
Stade nautique Rainier III     26
Fontaine Japonaise             18
Le Méridien • Pool             18
Fontaine du Casino             18


$ layer-stats variance openmaptiles.yml building render_height
======= Analyzing field value variance in layer 'building' for field [render_height] =======
zoom      count    min    max      avg    stddev    variance
------  -------  -----  -----  -------  --------  ----------
z14        1171      4    170  6.96328   11.1951     125.329