Bugfix improve example16

[rapoliveira]

It contains all the improvements to example 16 listed in issue #93.

As a complement, the value optimal is now allowed in "plots" -> "best model" -> "second Y scale" -> "magnifications", in order to calculate optimal values of magnification instead of giving values that can be out of range. If optimal is provided, labels can not be given for this axis.

[rapoliveira]

@rpoleski I have figured out some minor fixes in my previous commits, so I will work on it by the end of the week before you merge. Please also let me know if the version number should be updated.

[codecov-commenter]

Codecov Report

Merging #109 (6ad88e5) into master (828190b) will not change coverage.
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@@           Coverage Diff           @@
##           master     #109   +/-   ##
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  Coverage   83.48%   83.48%           
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  Files          52       52           
  Lines        8185     8185           
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  Hits         6833     6833           
  Misses       1352     1352           

Flag	Coverage Δ
unittests	83.48% <ø> (ø)

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[rpoleski]

Very final and minor changes:

• please change version number to 0.34.0
• Functions: _second_Y_axis_get_fluxes(), _second_Y_axis_optimal(), and _second_Y_axis_warnings() - please add a docstring just saying what they do (1 sentence is enough)
• in these functions, please change returns with multiple values: return A, B -> return (A, B)