Adjust Tuple{Vararg{LinearMap}} notation

[dkarrasch]

If I understand correctly, this is how it's supposed to be written after JuliaLang/julia#38136 (whose exact consequences I don't understand), and I was indeed seeing warnings during the tests on my local nightly (though not in the CI runs):

WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).
WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).
WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).

[codecov]

Codecov Report

Merging #132 (9250d09) into master (809ab02) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #132   +/-   ##
=======================================
  Coverage   99.44%   99.44%           
=======================================
  Files          14       14           
  Lines         906      906           
=======================================
  Hits          901      901           
  Misses          5        5           

Impacted Files	Coverage Δ
src/composition.jl	100.00% <100.00%> (ø)

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