fixing indexing of sigma matrix

[mmaelicke]

Closes #109

[codecov]

Codecov Report

Merging #110 (56f60b2) into master (2819ab4) will increase coverage by 0.02%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
+ Coverage   91.64%   91.66%   +0.02%     
==========================================
  Files          20       20              
  Lines        2035     2040       +5     
==========================================
+ Hits         1865     1870       +5     
  Misses        170      170              

Impacted Files	Coverage Δ
skgstat/Kriging.py	85.83% <89.47%> (+0.30%)	⬆️

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