Add converters to SnakeBidsApp

[pvandyken]

strings being passed to the arguments in old apps were breaking. Convert now to Paths

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.50%. Comparing base (ed7dbe6) to head (ad0b495).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #421   +/-   ##
=======================================
  Coverage   94.50%   94.50%           
=======================================
  Files          55       55           
  Lines        2184     2184           
=======================================
  Hits         2064     2064           
  Misses        120      120           

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