fix types multidim scaling

[arnaudvl]

Addresses #642 .

[codecov]

Codecov Report

Merging #646 (143a5a9) into master (3772a89) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
+ Coverage   80.39%   80.46%   +0.07%     
==========================================
  Files          77       77              
  Lines       10597    10635      +38     
==========================================
+ Hits         8519     8557      +38     
  Misses       2078     2078              

Impacted Files	Coverage Δ
alibi/explainers/cfproto.py	75.53% <100.00%> (ø)
alibi/utils/distance.py	98.11% <100.00%> (ø)
alibi/explainers/tests/test_ale.py	100.00% <0.00%> (ø)
alibi/api/interfaces.py	90.58% <0.00%> (+0.11%)	⬆️
alibi/explainers/ale.py	64.57% <0.00%> (+4.27%)	⬆️