Various fixes to MOI wrapper

[odow]

• support SolveTime
• work around maximumSeconds
• fix missing - sign in infeasibility certificates

Closes #65

[codecov-io]

Codecov Report

Merging #89 into master will increase coverage by 3.60%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
+ Coverage   48.87%   52.47%   +3.60%     
==========================================
  Files           3        3              
  Lines         399      404       +5     
==========================================
+ Hits          195      212      +17     
+ Misses        204      192      -12     

Impacted Files	Coverage Δ
src/moi/MOI_wrapper.jl	80.80% <91.66%> (+4.54%)	⬆️
src/gen/libclp_api.jl	16.85% <0.00%> (+1.68%)	⬆️

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