Cleanup nbconvert templates

[damianavila]

The templates have a lot of whitespaces, some missing ones... and there is no a pythonic structure when you read the code (some indentations missing an so on...)
So, I propose a clean up of the templates... I think this PR make them more readable and easy to understand for people who want to learn to write their own templates.
Maybe I am missing some details deleting white lines, so I open this PR to discuss and review the changes.

Cheers.

Damián.

[Carreau]

At the beginning the newlines /whitespace where to keep a perfect matching between nbconvert 1 and 2, so I have no opposition in cleaning them, and thanks a lot for that. So I don't have any objection for HTML ans Latex template,
I would just like to check on markdown and rst is the rendered document look natural and do not have too many extra line.

Travis build errored, I just relauched.

Tagging 1.0

[damianavila]

For reference, I have not touched the latex templates because I did not know if the current structure is necessary to render ok...
I can do reorganization of latex templates but I will need some eyes to see in depth ;-)

[damianavila]

OK, I added some fixes in latex templates to be congruent with the others... I did not have to do a lot of modifications because there was more organized than the non-latex templates.

[Carreau]

hum... need rebase. But you might want to wait for #3758 to land.

[damianavila]

I will rebase it in a couples of minutes... I am pushing you another PR ;-)

[damianavila]

@Carreau rebase done ;-)

[jdfreder]

Looks like it needs another rebase 😦

[damianavila]

I will do it in a couple of minutes

[damianavila]

Fixing rebasing conflicts, don't review yet... thanks.

[damianavila]

OK, I don know why travis is failing, probably it has to be run again... I am not familiarize with travis but I can run it again or some of you have to do it?

[minrk]

re-ran travis, wasn't an issue here. Personal testing, and this seems to be working fine for me. 👍 to merge, but I'll let @Carreau or @jdfreder push the big green button.

[jdfreder]

Good work @damianavila . I like these kinds of aesthetic changes :)

[damianavila]

Thanks!