Refactor code and split into many files

[abelsiqueira]

Closes #113

[dpo]

Thanks. I would suggest linalg.jl instead of factorization-operators.jl.

Also, BlockDiagonalOperator could go into special-operators.jl.

[coveralls]

Coverage remained the same at 96.308% when pulling fcff9b0 on abelsiqueira:refactor into 644cd34 on JuliaSmoothOptimizers:master.

[codecov]

Codecov Report

Merging #126 into master will not change coverage.
The diff coverage is 94.68%.

@@          Coverage Diff           @@
##           master    #126   +/-   ##
======================================
  Coverage    96.3%   96.3%           
======================================
  Files           8      14    +6     
  Lines         623     623           
======================================
  Hits          600     600           
  Misses         23      23

Impacted Files	Coverage Δ
src/lsr1.jl	98.48% <ø> (ø)	⬆️
src/LinearOperators.jl	100% <ø> (+5.59%)	⬆️
src/adjtrans.jl	98.71% <ø> (ø)	⬆️
src/lbfgs.jl	97.97% <ø> (ø)	⬆️
src/abstract.jl	100% <100%> (ø)
src/operations.jl	100% <100%> (ø)
src/linalg.jl	100% <100%> (ø)
src/cat.jl	100% <100%> (ø)
src/constructors.jl	72.72% <72.72%> (ø)
src/special-operators.jl	86.2% <86.2%> (ø)
... and 8 more

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[dpo]

Looks great, thank you!