minor updates to L-SR1 operator

[dpo]

I made mem a keyword arguments. The rest is all nitpicks.

[abelsiqueira]

The tests need to fixed as well.

[codecov]

Codecov Report

Merging #145 into master will increase coverage by 0.01%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
+ Coverage   96.32%   96.33%   +0.01%     
==========================================
  Files          14       14              
  Lines         625      627       +2     
==========================================
+ Hits          602      604       +2     
  Misses         23       23

Impacted Files	Coverage Δ
src/lbfgs.jl	98% <80%> (ø)	⬆️
src/lsr1.jl	98.55% <94.73%> (+0.04%)	⬆️
src/PreallocatedLinearOperators.jl	100% <0%> (ø)	⬆️

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[abelsiqueira]

Thanks