Switch default unit to millimetres for unitless formats

[alteous]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@e4a7772). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
modeling-cmds/src/format/obj.rs	0.00%	2 Missing ⚠️
modeling-cmds/src/format/ply.rs	0.00%	2 Missing ⚠️
modeling-cmds/src/format/stl.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #754   +/-   ##
=======================================
  Coverage        ?   25.20%           
=======================================
  Files           ?       35           
  Lines           ?     2107           
  Branches        ?        0           
=======================================
  Hits            ?      531           
  Misses          ?     1576           
  Partials        ?        0           

Flag	Coverage Δ
unittests	25.20% <0.00%> (?)

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