improved docs and interface, deprecated obsolete stuff

[HDembinski]

No description provided.

[codecov]

Codecov Report

Merging #374 into develop will increase coverage by 0.09%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #374      +/-   ##
===========================================
+ Coverage    91.19%   91.29%   +0.09%     
===========================================
  Files           17       17              
  Lines         1011     1011              
  Branches        18       18              
===========================================
+ Hits           922      923       +1     
+ Misses          79       78       -1     
  Partials        10       10

Impacted Files	Coverage Δ
iminuit/repr_text.py	98.94% <100%> (+0.01%)	⬆️
iminuit/_deprecated.py	100% <100%> (+10%)	⬆️
iminuit/repr_html.py	96.92% <100%> (-0.02%)	⬇️

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