The Stacks project

Slightly better handling of shift functors in triangulated categories

Thanks to Elías Guisado Villalgordo and Christian Q.

This closes #165. See discussion there for why I made the choices
I made in this commit

See
https://stacks.math.columbia.edu/tag/0144#comment-4327
https://stacks.math.columbia.edu/tag/0144#comment-4483
https://stacks.math.columbia.edu/tag/05R6#comment-8343

Minor corrections on derived.tex. The most important one is a clarification of TR3, which confused me (I thought both the left square and the right long rectangle were assumed commutative).

New chapter on derived categories

On triangulated categories

	Small changes to prepare adding a chapter on derived categories.

Latex: get rid of some overfull boxes

	modified:   algebraic.tex
	modified:   chow.tex
	modified:   coherent.tex
	modified:   cohomology.tex
	modified:   constructions.tex
	modified:   descent.tex
	modified:   desirables.tex
	modified:   divisors.tex
	modified:   etale.tex
	modified:   examples-stacks.tex
	modified:   examples.tex
	modified:   exercises.tex
	modified:   fdl.tex
	modified:   homology.tex
	modified:   hypercovering.tex
	modified:   modules.tex
	modified:   morphisms.tex
	modified:   properties.tex
	modified:   schemes.tex
	modified:   simplicial.tex
	modified:   sites.tex
	modified:   spaces.tex
	modified:   topologies.tex
	modified:   varieties.tex

Started tags infrastructure

	new file:   scripts/add_tags.py
	modified:   scripts/functions.py
	new file:   tags/initial_tags
	new file:   tags/tags

Reorganization of material in homology.tex

	modified:   homology.tex

K(A) is a triangulated category

	modified:   homology.tex
	modified:   my.bib

Working towards TR4 in K(A)

	modified:   homology.tex

Triangle, triangle on the wall...

	modified:   homology.tex
	modified:   my.bib