Reengineer Tuple to not store empty types

[bernhardmgruber]

No description provided.

[codecov-commenter]

Codecov Report

Merging #436 (35e21fb) into develop (48b468d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #436   +/-   ##
========================================
  Coverage    99.22%   99.22%           
========================================
  Files           56       56           
  Lines         5130     5150   +20     
========================================
+ Hits          5090     5110   +20     
  Misses          40       40           

Impacted Files	Coverage Δ
include/llama/Tuple.hpp	100.00% <100.00%> (ø)
include/llama/mapping/tree/Functors.hpp	97.77% <100.00%> (-0.05%)	⬇️
include/llama/mapping/tree/Mapping.hpp	100.00% <100.00%> (ø)
include/llama/mapping/tree/toString.hpp	100.00% <100.00%> (ø)
tests/tuple.cpp	100.00% <100.00%> (ø)

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