BUG: undef values in grdecl export shall be 0 or 1

[jcrivenaes]

Closes #1279

Note: It could also be possible to let the user input this value, but that will be a larger change. I propose we add this a patch now and consider future improvements later.

Related to this is also the use of the isdiscrete property and dtype option in the to_file() appears unclear for me now. Perhaps a need for a deeper look into this at some point.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.15%. Comparing base (0375fed) to head (28f6930).
Report is 53 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1280      +/-   ##
==========================================
+ Coverage   80.02%   81.15%   +1.12%     
==========================================
  Files          98       94       -4     
  Lines       13680    12436    -1244     
  Branches     2203     1874     -329     
==========================================
- Hits        10948    10092     -856     
+ Misses       1999     1685     -314     
+ Partials      733      659      -74     

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