Fix residual computation

[mtanneau]

Closes #106

[codecov-commenter]

Codecov Report

Merging #115 (b89f032) into master (1098b00) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #115   +/-   ##
=======================================
  Coverage   89.00%   89.00%           
=======================================
  Files          43       43           
  Lines        2756     2756           
=======================================
  Hits         2453     2453           
  Misses        303      303           

Impacted Files	Coverage Δ
src/IPM/HSD/HSD.jl	87.50% <100.00%> (ø)
src/IPM/MPC/MPC.jl	87.50% <100.00%> (ø)

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