Implement general case non-mutating apply

[nicoleepp]

Part of #12

[codecov]

Codecov Report

Merging #15 (c0cff1a) into main (2e23f97) will decrease coverage by 7.57%.
The diff coverage is 93.75%.

@@             Coverage Diff             @@
##              main      #15      +/-   ##
===========================================
- Coverage   100.00%   92.42%   -7.58%     
===========================================
  Files            6        6              
  Lines           69       66       -3     
===========================================
- Hits            69       61       -8     
- Misses           0        5       +5     

Impacted Files	Coverage Δ
src/transformers.jl	95.23% <90.00%> (-4.77%)	⬇️
src/periodic.jl	100.00% <100.00%> (ø)
src/power.jl	100.00% <100.00%> (ø)
src/utils.jl	0.00% <0.00%> (-100.00%)	⬇️
src/linear_combination.jl	100.00% <0.00%> (ø)

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[bencottier]

Looks good, I just have a few suggestions and uncertainties.