make standardize_metadata more helpful

[kmdalton]

When metadata columns have zero variance, it is helpful to print the column key.

[kmdalton]

this resolves #131

[codecov-commenter]

Codecov Report

Merging #132 (89050fd) into main (bd1652b) will increase coverage by 0.52%. Report is 7 commits behind head on main. The diff coverage is 63.63%.

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
+ Coverage   78.51%   79.03%   +0.52%     
==========================================
  Files          50       52       +2     
  Lines        2220     2347     +127     
==========================================
+ Hits         1743     1855     +112     
- Misses        477      492      +15     

Flag	Coverage Δ
unittests	79.03% <63.63%> (+0.52%)	:arrow_up:

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Files Changed	Coverage Δ
careless/io/formatter.py	90.72% <63.63%> (-0.64%)	:arrow_down:

... and 2 files with indirect coverage changes

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[DHekstra]

Thank you