added possibility to scale single terms

[j-moser]

• when scaling single terms, the scale function does now substitute redundant averages
• added get_scale_solution to get a solution of an average which is scale-equal to one of the system states
• added test cases

[codecov[bot]]

Codecov Report

Merging #163 (ae9b4dc) into master (546ca6d) will decrease coverage by 0.06%. The diff coverage is 45.16%.

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
- Coverage   81.05%   81.00%   -0.06%     
==========================================
  Files          24       24              
  Lines        4445     4475      +30     
==========================================
+ Hits         3603     3625      +22     
- Misses        842      850       +8     

Impacted Files	Coverage Δ
src/QuantumCumulants.jl	100.00% <ø> (ø)
src/utils.jl	90.68% <0.00%> (-5.65%)	:arrow_down:
src/index_average.jl	82.62% <100.00%> (+0.32%)	:arrow_up:
src/index_scale.jl	64.92% <100.00%> (+4.07%)	:arrow_up:
src/index_utils.jl	94.11% <100.00%> (+0.78%)	:arrow_up:

... and 2 files with indirect coverage changes

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