Add Variable map

[blegat]

Extracted from #759

[codecov-io]

Codecov Report

Merging #813 into master will decrease coverage by 1.75%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #813      +/-   ##
==========================================
- Coverage   93.87%   92.12%   -1.76%     
==========================================
  Files          62       63       +1     
  Lines        6943     7075     +132     
==========================================
  Hits         6518     6518              
- Misses        425      557     +132

Impacted Files	Coverage Δ
src/Bridges/Variable/map.jl	0% <0%> (ø)

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[codecov-io]

Codecov Report

Merging #813 into master will decrease coverage by 1.77%.
The diff coverage is 0.74%.

@@            Coverage Diff            @@
##           master    #813      +/-   ##
=========================================
- Coverage   93.87%   92.1%   -1.78%     
=========================================
  Files          62      64       +2     
  Lines        6943    7078     +135     
=========================================
+ Hits         6518    6519       +1     
- Misses        425     559     +134

Impacted Files	Coverage Δ
src/Bridges/Variable/bridge.jl	0% <ø> (ø)	⬆️
src/Bridges/Bridges.jl	100% <ø> (ø)	⬆️
src/Bridges/Variable/map.jl	0% <0%> (ø)
src/Utilities/functions.jl	94.98% <0%> (-0.36%)	⬇️
src/Bridges/Variable/Variable.jl	100% <100%> (ø)

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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