Add Var.slice_intp

[ph-kev]

This PR adds slicing by using the interpolant of the OutputVar.

Helps with issue #165

[codecov]

Codecov Report

❌ Patch coverage is 69.56522% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.82%. Comparing base (ecd089a) to head (f5d7fed).
⚠️ Report is 145 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Var.jl	69.56%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
- Coverage   98.35%   97.82%   -0.54%     
==========================================
  Files          11       11              
  Lines        1218     1241      +23     
==========================================
+ Hits         1198     1214      +16     
- Misses         20       27       +7     

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[Sbozzolo]

I think that this should be a keyword argument in slice