Bug fix for TrustRegion when iip=true.

[Deltadahl]

This is an attempt to fix the bug that currently exists in the TrustRegion solver when iip=true #124.
The bug went trough testing because the tests did not take the absolute value, allowing all negative values to pass, i.e. @test all(sol.u .* sol.u .- 2 .< 1e-9) and not @test all(abs.(sol.u .* sol.u .- 2) .< 1e-9) as it should be.

The bug is due to that cache.u and cache.fu points to the same vector as cache.u_tmp and cache.fu_new respectively, and then both of them get changed at unwanted places.

[codecov]

Codecov Report

Merging #125 (69350aa) into master (9049a3c) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   89.21%   89.36%   +0.15%     
==========================================
  Files           6        6              
  Lines         343      348       +5     
==========================================
+ Hits          306      311       +5     
  Misses         37       37              

Impacted Files	Coverage Δ
src/trustRegion.jl	98.30% <100.00%> (+0.07%)	⬆️

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