Move pad_number() to strtools

[lguerard]

Fixes #58

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 24.98%. Comparing base (2027558) to head (ef64219).
Report is 14 commits behind head on devel.

Files with missing lines	Patch %	Lines
src/imcflibs/imagej/misc.py	0.00%	1 Missing ⚠️
src/imcflibs/strtools.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            devel      #64   +/-   ##
=======================================
  Coverage   24.98%   24.98%           
=======================================
  Files          25       25           
  Lines        1661     1661           
=======================================
  Hits          415      415           
  Misses       1246     1246           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.