Use align_variable_duration_unit function instead of convert_to_days

[datejada]

Reusing the function align_variable_duration_unit instead of creating a new one convert_to_days

Fixes #1152

Checklist before merging

• Documentation is up-to-date
• Unit tests have been added/updated accordingly
• Code has been formatted according to SpineOpt's style
• Unit tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.39%. Comparing base (f6bf05a) to head (d68fe91).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1154      +/-   ##
==========================================
+ Coverage   87.35%   87.39%   +0.03%     
==========================================
  Files         143      143              
  Lines        4135     4132       -3     
==========================================
- Hits         3612     3611       -1     
+ Misses        523      521       -2     

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[datejada]

@nnhjy your function does the same as the one I added before. So, I am reusing yours since it is better documented and to avoid having dirty hacks inside the functions.
Please let me know if you have any comments, otherwise, I will merge this change.
Thanks!