interp_relperm Allow two phase interpolation

[alifbe]

Closes #168

[mferrera]

Approving this to allow for merging since the changes are not numerous, but I of course cannot comment on correctness.

Just a leftover print statement to be removed 😄

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.55%. Comparing base (f56fde2) to head (1c1ea64).
Report is 10 commits behind head on main.

❗ Current head 1c1ea64 differs from pull request most recent head 84edf34

Please upload reports for the commit 84edf34 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #701      +/-   ##
==========================================
- Coverage   85.69%   85.55%   -0.15%     
==========================================
  Files          50       49       -1     
  Lines        7005     6577     -428     
==========================================
- Hits         6003     5627     -376     
+ Misses       1002      950      -52     

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