Use nbviewer for linking notebooks.

[gerritholl]

When linking to jupyter notebooks from the Satpy documentation, add a
direct link to nbviewer rather than to Github. Github often fails to
render notebooks.

• Fully documented

[coveralls]

Coverage increased (+0.003%) to 90.136% when pulling 02cd737 on gerritholl:use-nbviewer into b0d8460 on pytroll:master.

[codecov]

Codecov Report

Merging #1325 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1325   +/-   ##
=======================================
  Coverage   90.13%   90.13%           
=======================================
  Files         220      220           
  Lines       32512    32512           
=======================================
  Hits        29305    29305           
  Misses       3207     3207           

---

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[mraspaud]

Did you check that they all render correctly in nbviewer? Iirc I had cases where Github would render better than nbviewer, depending on where the images we're hosted

[gerritholl]

No, I didn't check if they all render correctly in nbviewer. Considering that none of them render anything in Github at all, any rendering is better than none.

[gerritholl]

• https://nbviewer.jupyter.org/github/pytroll/pytroll-examples/blob/master/satpy/hrit_msg_tutorial.ipynb
• https://nbviewer.jupyter.org/github/pytroll/pytroll-examples/blob/master/satpy/Cartopy%20Plot.ipynb
• https://nbviewer.jupyter.org/github/pytroll/pytroll-examples/blob/master/satpy/ahi_true_color_pyspectral.ipynb
• https://nbviewer.jupyter.org/github/pytroll/pytroll-examples/blob/master/satpy/OLCI%20L1B.ipynb
• https://nbviewer.jupyter.org/github/pytroll/pytroll-examples/blob/master/satpy/Sentinel%202%20MSI%20true%20color.ipynb
• https://nbviewer.jupyter.org/github/pytroll/pytroll-examples/blob/master/satpy/satpy_rayleigh_iband_enhanced.ipynb
• https://nbviewer.jupyter.org/github/pytroll/pytroll-examples/blob/master/satpy/satpy_rayleigh_modis.ipynb
• https://nbviewer.jupyter.org/github/pytroll/pytroll-examples/blob/master/satpy/sentinel1-false-color.ipynb
• https://nbviewer.jupyter.org/github/pytroll/pytroll-examples/blob/master/satpy/ears-nwc.ipynb
• https://nbviewer.jupyter.org/github/pytroll/pytroll-examples/blob/master/satpy/polar_maia.ipynb

Images appear to fail to render for most of them, but that's still better than no rendering at all. Otherwise perhaps there is a way to generate static HTML, add those to the Pytroll website and link those instead?

[mraspaud]

We've discussed the possibility of a generated gallery indeed, but haven't found a solution yet

[gerritholl]

Until we have that, I think viewing the notebooks with most images missing (such as this PR) is still an improvement to not being able to view them at all (in my experience, Github viewing of notebooks almost never works).