Merge RFT data on report_step in addition to well and order

[alifbe]

For observation with multiple report steps, it is necessary to also merge on report_step

Resolve #455

[codecov-commenter]

Codecov Report

Merging #619 (5d0ed91) into main (469a6ec) will increase coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head 5d0ed91 differs from pull request most recent head ae6495d. Consider uploading reports for the commit ae6495d to get more accurate results

@@            Coverage Diff             @@
##             main     #619      +/-   ##
==========================================
+ Coverage   84.72%   84.74%   +0.01%     
==========================================
  Files          52       52              
  Lines        7510     7517       +7     
==========================================
+ Hits         6363     6370       +7     
  Misses       1147     1147              

Files	Coverage Δ
src/subscript/merge_rft_ertobs/merge_rft_ertobs.py	95.06% <100.00%> (+0.46%)	⬆️

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[rnyb]

LGTM