Simplify dijkstra code and add tests

[dramanica]

Fix #19

[codecov]

Codecov Report

Attention: Patch coverage is 92.50000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 69.51%. Comparing base (d0ac91d) to head (04dbe1e).
Report is 3 commits behind head on dev.

Files	Patch %	Lines
R/connectivity.R	75.00%	1 Missing ⚠️
R/dijkstra.R	85.71%	1 Missing ⚠️
R/hasCosts.R	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #21      +/-   ##
==========================================
+ Coverage   65.43%   69.51%   +4.07%     
==========================================
  Files          19       21       +2     
  Lines        1325     1335      +10     
==========================================
+ Hits          867      928      +61     
+ Misses        458      407      -51     

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