Pass dir paths to download functions

[leonlan]

Closes #64.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c4faece) 100.00% compared to head (664df26) 100.00%.

❗ Current head 664df26 differs from pull request most recent head 41c07a3. Consider uploading reports for the commit 41c07a3 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #87   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines          231       241   +10     
=========================================
+ Hits           231       241   +10     

Impacted Files	Coverage Δ
vrplib/download/download_instance.py	100.00% <100.00%> (ø)
vrplib/download/download_solution.py	100.00% <100.00%> (ø)

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