add missing methods and fix tests

[dpo]

supersedes #10

[codecov[bot]]

Codecov Report

Merging #13 into master will increase coverage by 25.35%. The diff coverage is 75%.

@@             Coverage Diff             @@
##           master      #13       +/-   ##
===========================================
+ Coverage   40.67%   66.03%   +25.35%     
===========================================
  Files           3        3               
  Lines          59       53        -6     
===========================================
+ Hits           24       35       +11     
+ Misses         35       18       -17

Impacted Files	Coverage Δ
src/qpmodel.jl	66% <75%> (+26.71%)	:arrow_up:

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[abelsiqueira]

Thank you