make iterate configurable

[HDembinski]

@matthewfeickert @kratsg This patch makes the iteration ("try hard") feature of iminuit configurable. You should definitely try to play with the precision keyword, too, because I did and it improve convergence when the cost function has low precision (the doc string was extended to better explain what this setting does).

[codecov]

Codecov Report

Merging #488 into develop will increase coverage by 0.79%.
The diff coverage is 97.01%.

@@             Coverage Diff             @@
##           develop     #488      +/-   ##
===========================================
+ Coverage    94.50%   95.30%   +0.79%     
===========================================
  Files           27       31       +4     
  Lines         2240     2937     +697     
  Branches        16        0      -16     
===========================================
+ Hits          2117     2799     +682     
- Misses         114      138      +24     
+ Partials         9        0       -9     

Impacted Files	Coverage Δ
src/iminuit/IMinuitMixin.h	100.00% <ø> (ø)
src/iminuit/PythonCaller.h	100.00% <ø> (+8.88%)	⬆️
src/iminuit/PythonFCN.h	100.00% <ø> (ø)
src/iminuit/__init__.py	66.66% <ø> (ø)
src/iminuit/_deprecated.py	100.00% <ø> (ø)
src/iminuit/color.py	100.00% <ø> (ø)
src/iminuit/info.py	0.00% <0.00%> (ø)
src/iminuit/tests/test_color.py	100.00% <ø> (ø)
src/iminuit/tests/test_describe.py	68.47% <78.94%> (+2.26%)	⬆️
src/iminuit/util.py	86.17% <85.80%> (+1.71%)	⬆️
... and 45 more

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[matthewfeickert]

This patch makes the iteration ("try hard") feature of iminuit configurable. You should definitely try to play with the precision keyword, too, because I did and it improve convergence when the cost function has low precision (the doc string was extended to better explain what this setting does).

Fantastic! Many thanks @HDembinski and we'll get on that (I'll make a follow up Issue on pyhf).