R2N: implicitely update the linear operator

[MaxenceGollier]

#231

[MaxenceGollier]

@dpo, in TR and TRDH, we do it like this:

Bk =
    isa(reg_nlp.model, QuasiNewtonModel) ? hess_op(reg_nlp.model, xk) :
    hess_op!(reg_nlp.model, xk, similar(xk))

but I am not sure to understand why.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.66%. Comparing base (e0f214d) to head (3cbcf51).
⚠️ Report is 259 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #274       +/-   ##
===========================================
+ Coverage   61.53%   84.66%   +23.13%     
===========================================
  Files          11       13        +2     
  Lines        1292     1624      +332     
===========================================
+ Hits          795     1375      +580     
+ Misses        497      249      -248     

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