Fix missing _to_mat function

[glennmoy]

Follow up to #61, I missed this during the rebase.

[codecov]

Codecov Report

Merging #66 (b8ca1d4) into main (271e3f5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #66   +/-   ##
=======================================
  Coverage   99.12%   99.12%           
=======================================
  Files          10       10           
  Lines         114      114           
=======================================
  Hits          113      113           
  Misses          1        1           

Impacted Files	Coverage Δ
src/apply.jl	100.00% <100.00%> (ø)
src/linear_combination.jl	100.00% <100.00%> (ø)
src/scaling.jl	100.00% <100.00%> (ø)

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