Filter out explicit zeros in new rows/columns

[mtanneau]

Adds checks on new rows/columns, namely:

• zero coefficients are filtered out
• an error is thrown if a coefficient is not finite

It also fixes a bug in add_variable! that had somehow gone undetected.
Thanks @salomonl for bringing this to light!

Closes #77
cc @amontoison

[codecov-io]

Codecov Report

Merging #79 (060f2a2) into master (5066f8b) will decrease coverage by 0.12%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
- Coverage   89.91%   89.78%   -0.13%     
==========================================
  Files          36       36              
  Lines        2528     2408     -120     
==========================================
- Hits         2273     2162     -111     
+ Misses        255      246       -9     

Impacted Files	Coverage Δ
src/problemData.jl	92.20% <94.44%> (+6.49%)	⬆️
src/KKT/KKT.jl	77.77% <0.00%> (-2.23%)	⬇️
src/Interfaces/tulip_julia_api.jl	67.74% <0.00%> (-1.96%)	⬇️
src/Interfaces/MOI/attributes.jl	65.45% <0.00%> (-1.79%)	⬇️
src/Interfaces/MOI/variables.jl	87.80% <0.00%> (-1.33%)	⬇️
src/KKT/krylov.jl	89.02% <0.00%> (-1.30%)	⬇️
src/Interfaces/MOI/objective.jl	90.00% <0.00%> (-1.18%)	⬇️
src/Interfaces/MOI/constraints.jl	89.14% <0.00%> (-1.10%)	⬇️
src/Presolve/forcing_row.jl	70.47% <0.00%> (-0.83%)	⬇️
src/Interfaces/MOI/MOI_wrapper.jl	80.35% <0.00%> (-0.68%)	⬇️
... and 21 more

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[amontoison]

Great, thanks @mtanneau !