Add dim_keys for ModelInstance

[lrennels]

@davidanthoff #766

[codecov]

Codecov Report

Merging #767 into master will decrease coverage by 0.52%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #767      +/-   ##
==========================================
- Coverage   78.74%   78.21%   -0.53%     
==========================================
  Files          35       37       +2     
  Lines        2973     2993      +20     
==========================================
  Hits         2341     2341              
- Misses        632      652      +20     

Flag	Coverage Δ
#unittests	78.21% <16.66%> (-0.53%)	⬇️

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Impacted Files	Coverage Δ
src/core/model.jl	82.85% <ø> (ø)
src/mcs/delta.jl	0.00% <0.00%> (ø)
src/mcs/mcs_utils.jl	100.00% <100.00%> (ø)
src/mcs/sobol.jl	88.46% <100.00%> (-0.43%)	⬇️

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