Fix value_type and key_type for Tuple

[blegat]

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[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 6 lines in your changes missing coverage. Please review.

Project coverage is 85.93%. Comparing base (ba3697f) to head (7836f8e).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/coefficients.jl	40.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
- Coverage   86.26%   85.93%   -0.33%     
==========================================
  Files          14       14              
  Lines         757      761       +4     
==========================================
+ Hits          653      654       +1     
- Misses        104      107       +3     

Flag	Coverage Δ
unittests	85.93% <71.42%> (-0.33%)	⬇️

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[blegat]

@kalmarek Are you still ok now ?
For consistency with key_type, I changed everything to value_type. It also avoids having a code using valtype that works for everything except tuples as it forces people to use value_type otherwise it won't work for much.

[blegat]

We could also require using StaticArrays.SVector instead of tuples but the changes needed for tuples seem easy enough. SparseCoefficients are already weird enough so tuples aren't such freaks