Cleanup backlog issues

[joaquimg]

fixed #91
tested locally

close #99
close #98
close #90
close #92

[codecov]

Codecov Report

Merging #100 into master will decrease coverage by 0.28%.
The diff coverage is 42.97%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
- Coverage   59.05%   58.77%   -0.29%     
==========================================
  Files           7        7              
  Lines        2528     2559      +31     
==========================================
+ Hits         1493     1504      +11     
- Misses       1035     1055      +20     

Impacted Files	Coverage Δ
src/Xpress.jl	100.00% <ø> (ø)
src/lib.jl	10.04% <0.00%> (-0.01%)	⬇️
src/MOI/MOI_wrapper.jl	86.62% <56.96%> (-0.65%)	⬇️
src/helper.jl	41.00% <77.77%> (-0.59%)	⬇️
src/api.jl	25.18% <0.00%> (+0.18%)	⬆️

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