Fix: units pm and mm missing from spm_pixel_to_nm_scaling

[SylviaWhittle]

Closes #83

Adds the same units dictionary that was used in TopoStats to AFMReader. Apologies, I somehow omitted the units pm and mm from when I copied the code over.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.81%. Comparing base (f282c31) to head (1821f51).
Report is 198 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
+ Coverage   65.87%   71.81%   +5.94%     
==========================================
  Files           3        8       +5     
  Lines         337      589     +252     
==========================================
+ Hits          222      423     +201     
- Misses        115      166      +51     

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[ns-rse]

👍 quick work.

The only thing I can think of is whether we have tests in place to ensure these units are handled and also correctly handle unsupported units (e.g. cm or m and I mention the later because @tcatley pull request to report the mean width suggests the unit this is returned in is metres, see line 276 of Pull Request #999)