Fix non-functional stackdates

[berland]

This fixes an ugly but that has been reported by an asset user, and probably warrants pushing into komodo asap.

The code that has been touched is apparently ugly, and could be a candidate for further refactoring.

[codecov-commenter]

Codecov Report

Merging #329 (46a482b) into master (499b041) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 46a482b differs from pull request most recent head 4cf63c8. Consider uploading reports for the commit 4cf63c8 to get more accurate results

@@           Coverage Diff           @@
##           master     #329   +/-   ##
=======================================
  Coverage   90.28%   90.28%           
=======================================
  Files          22       22           
  Lines        3025     3027    +2     
=======================================
+ Hits         2731     2733    +2     
  Misses        294      294           

Impacted Files	Coverage Δ
ecl2df/grid.py	86.61% <100.00%> (+0.10%)	⬆️

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