Make LinearMaps callable

[dkarrasch]

Closes #106.

[codecov]

Codecov Report

Merging #128 (32078f9) into master (ef64037) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #128   +/-   ##
=======================================
  Coverage   99.44%   99.44%           
=======================================
  Files          14       14           
  Lines         905      906    +1     
=======================================
+ Hits          900      901    +1     
  Misses          5        5           

Impacted Files	Coverage Δ
src/kronecker.jl	100.00% <ø> (ø)
src/LinearMaps.jl	97.36% <100.00%> (+0.03%)	⬆️

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