Copy parts of grdecl_io to heat-eq for speed

[dafeda]

Importing ert in forward model increases ES-MDA run from 50s to 2min 30s.

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure tests pass locally (after every commit!)

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[eivindjahren]

👍

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.65%. Comparing base (1336b7c) to head (3594762).
Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8488      +/-   ##
==========================================
+ Coverage   90.64%   90.65%   +0.01%     
==========================================
  Files         345      345              
  Lines       20981    20981              
==========================================
+ Hits        19018    19021       +3     
+ Misses       1963     1960       -3     

Flag	Coverage Δ
gui-tests	75.92% <ø> (-0.02%)	⬇️
integration-tests	53.94% <ø> (+<0.01%)	⬆️
unit-tests	68.66% <ø> (+0.02%)	⬆️

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