Allow wrapping of vectors, extend Kronecker product

[dkarrasch]

This treats vectors upon wrapping in LinearMap like a column matrix, without reshaping however. This allows to easily represent rank-1 operators as v ⊗ w' with optimal performance.

[codecov]

Codecov Report

Merging #136 (6cfda4f) into master (209d6e2) will increase coverage by 0.00%.
The diff coverage is 97.43%.

@@           Coverage Diff           @@
##           master     #136   +/-   ##
=======================================
  Coverage   99.80%   99.80%           
=======================================
  Files          14       14           
  Lines        1037     1042    +5     
=======================================
+ Hits         1035     1040    +5     
  Misses          2        2           

Impacted Files	Coverage Δ
src/LinearMaps.jl	97.67% <83.33%> (ø)
src/blockmap.jl	100.00% <100.00%> (ø)
src/conversion.jl	100.00% <100.00%> (ø)
src/kronecker.jl	100.00% <100.00%> (ø)
src/show.jl	100.00% <100.00%> (ø)
src/wrappedmap.jl	100.00% <100.00%> (ø)

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[dkarrasch]

Are there any objections against starting to treat vectors as column matrices in LinearMaps constructors? If not, I think this is a nice generalization, and there was a request for that in the Slack channel for lazy outer products.

[Jutho]

I guess that is indeed fine; I see no obstructions.