Xyz writer optionally converts vector units

[bhourahine]

Reverts the effect of ae1701a on the modes eigenvectors (which were being converted as though they were velocities).

[codecov]

Codecov Report

Attention: Patch coverage is 52.94118% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 70.81%. Comparing base (84e04ec) to head (8c149a8).

Files	Patch %	Lines
src/dftbp/io/formatout.F90	56.25%	7 Missing ⚠️
app/modes/modes.F90	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1426      +/-   ##
==========================================
- Coverage   70.82%   70.81%   -0.01%     
==========================================
  Files         231      231              
  Lines       44110    44107       -3     
==========================================
- Hits        31239    31236       -3     
  Misses      12871    12871              

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