Implement unary - and scalar product for Adjoint and Transpose

[abelsiqueira]

Closes #107

[coveralls]

Coverage increased (+1.6%) to 91.527% when pulling 3635cf7 on abelsiqueira:fix/issue107 into 96aac23 on JuliaSmoothOptimizers:master.

[codecov]

Codecov Report

Merging #108 into master will increase coverage by 1.09%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   90.96%   92.06%   +1.09%     
==========================================
  Files           6        6              
  Lines         520      529       +9     
==========================================
+ Hits          473      487      +14     
+ Misses         47       42       -5

Impacted Files	Coverage Δ
src/adjtrans.jl	79.62% <75%> (+12.23%)	⬆️
src/LinearOperators.jl	92.88% <0%> (+0.42%)	⬆️

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[dpo]

Thank you!