Bugfix example16: minor changes to magnification axis

[rapoliveira]

Minor changes were applied to the magnification second Y-axis, as suggested by Jason. I changed the color to 'black', label to 'Magnifications' and ticks to 'inside'.

Minor ticks (as in issue #118) were left for the future, since the axis scale is neither linear or log.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.52%. Comparing base (6fabea2) to head (959f353).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #120 +/- ## ======================================= Coverage 83.52% 83.52% ======================================= Files 52 52 Lines 8318 8318 ======================================= Hits 6948 6948 Misses 1370 1370 ``` | [Flag](https://app.codecov.io/gh/rpoleski/MulensModel/pull/120/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Radek+Poleski) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/rpoleski/MulensModel/pull/120/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Radek+Poleski) | `83.52% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Radek+Poleski#carryforward-flags-in-the-pull-request-comment) to find out more.

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[rpoleski]

Please remove # , minor=True) due to Jen's rule no. 1: "Don't leave unused code".

Also please change version to 0.34.4.

[rapoliveira]

@rpoleski To make sure I don't forget and since this second Y-axis has a different scale, I created a small function to handle its minor ticks. I already changed the version, but I'm open to suggestions!

[rpoleski]

A few more changes are needed:

• _mark_second_Y_axis_in_best_plot() got quite long, so please split it into shorter functions (Jen's rule no. 2: "Functions should be shorter than 20 lines.",
• name of variable sb_fluxes is unclear to me (and not explained in docstings) - please modify it.

[rapoliveira]

@rpoleski Done!

[rpoleski]

sb_fluxes -> ref_fluxes in _second_Y_axis_minor_ticks() and it will be finished.