Add edges pointing from higher ATC levels to lower ATC levels (e.g. ATC2 --> ATC3)

[DilumAluthge]

No description provided.

[codecov]

Codecov Report

Merging #10 (7ecf3ea) into master (a080f14) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #10   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        11    +1     
  Lines          237       290   +53     
=========================================
+ Hits           237       290   +53     

Impacted Files	Coverage Δ
src/PharmaceuticalClassification.jl	100.00% <ø> (ø)
src/iterate.jl	100.00% <ø> (ø)
src/types.jl	100.00% <ø> (ø)
src/vertex.jl	100.00% <ø> (ø)
src/atc_fix.jl	100.00% <100.00%> (ø)
src/build.jl	100.00% <100.00%> (ø)
src/system.jl	100.00% <100.00%> (ø)
src/traversal.jl	100.00% <100.00%> (ø)

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