Add example stub for unit tests and stop download failures from breaking unit tests

[mmiller-max]

• Add stubbed OSMGraph that can be used for unit tests without internet connection
• Use stub throughout tests
• Allow status errors to occur when downloading graphs (occurs in tests occasionally)

[codecov-commenter]

Codecov Report

Merging #41 (1604a28) into master (f1be1a9) will increase coverage by 1.57%.
The diff coverage is n/a.

❗ Current head 1604a28 differs from pull request most recent head 794a73e. Consider uploading reports for the commit 794a73e to get more accurate results

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   67.10%   68.67%   +1.57%     
==========================================
  Files          11       11              
  Lines         836      878      +42     
==========================================
+ Hits          561      603      +42     
  Misses        275      275              

Impacted Files	Coverage Δ
src/geometry.jl	100.00% <ø> (ø)
src/types.jl	61.53% <0.00%> (-5.13%)	⬇️
src/download.jl	57.64% <0.00%> (-1.39%)	⬇️
src/traversal.jl	60.41% <0.00%> (-0.95%)	⬇️
src/parse.jl	74.33% <0.00%> (-0.67%)	⬇️
src/buildings.jl	0.00% <0.00%> (ø)
src/constants.jl	100.00% <0.00%> (ø)
src/utilities.jl	98.11% <0.00%> (+0.09%)	⬆️
src/shortest_path.jl	87.50% <0.00%> (+0.65%)	⬆️
... and 1 more

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[captchanjack]

LGTM