ALE custom grid points update.

[RobertSamoilescu]

Changes behavior from:

Grid points outside the feature range. Consider the following example: `O O O X X O X O X O O`, \
where 3 grid-points are smaller than the minimum value in `f`, and 2 grid-points are larger than the maximum \
value in `f`. Grid-points outside the feature value range are clipped between the minimum and maximum \
values of `f`. The grid-points considered will be: `(O|X) X O X O (X|O)`.

to:

Grid points outside the feature range. Consider the following example: `O O O X X O X O X O O`, \
where 3 grid-points are smaller than the minimum value in `f`, and 2 grid-points are larger than the maximum \
value in `f`. The empty leading and ending bins are removed. The grid-points considered
will be: `O X X O X O X O`.

Note that the new approach is less strict in terms of the lower & upper bound of the grid. This further allows
custom lower/upper grid values to be considered (previously the grid points were clamped to the min/max feature values) for which the ALE is still well defined.

The PR also includes a stress test for custom grid points.

[codecov]

Codecov Report

Merging #731 (1f89be4) into master (dc55aba) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #731      +/-   ##
==========================================
+ Coverage   81.06%   81.12%   +0.05%     
==========================================
  Files         105      105              
  Lines       11794    11828      +34     
==========================================
+ Hits         9561     9595      +34     
  Misses       2233     2233              

Impacted Files	Coverage Δ
alibi/explainers/ale.py	65.65% <100.00%> (+1.07%)	⬆️
alibi/explainers/tests/test_ale.py	100.00% <100.00%> (ø)

[jklaise]

Nice!

[jklaise]

were -> are