Adds cubic smoothing of Mualem relative permeability

[bishtgautam]

• Cubic smoothing is added for Mualem relative permeability. The approach implemented here follows the [same approach] taken by PFLOTRAN.
• New variables are defined so variables are not shared between different relative perm and saturation functions
• Few regression files are updated due to the changes caused by cubic interpolation.

[codecov-commenter]

Codecov Report

Merging #193 (fefb1c6) into master (c110a28) will not change coverage. The diff coverage is n/a.

:exclamation: Current head fefb1c6 differs from pull request most recent head cd522df. Consider uploading reports for the commit cd522df to get more accurate results

@@           Coverage Diff           @@
##           master     #193   +/-   ##
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  Coverage   67.73%   67.73%           
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  Files           7        7           
  Lines        1221     1221           
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  Hits          827      827           
  Misses        394      394           

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[bishtgautam]

@jeff-cohere I added one more commit to address your comments.