Make `nlp` a concretely typed field in `MadNLPSolver`

[baggepinnen]

Before this PR, all accesses to fields on ips.nlp were type unstable and caused allocations.

[codecov-commenter]

Codecov Report

Merging #220 (bb93607) into master (ff28e44) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   74.33%   74.33%           
=======================================
  Files          37       37           
  Lines        3674     3674           
=======================================
  Hits         2731     2731           
  Misses        943      943           

Impacted Files	Coverage Δ
src/IPM/IPM.jl	98.75% <100.00%> (ø)

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