Removed admits from LongExactSequences.v
Moved lemma from Pullbacks3 to Auxiliary.
Pullbacks3: removed redundant [compose_cospan_cones], replaced with [left_cospan_cone_to_composite] throughout.
Added [cospan_cone_path'_map1] and [_map2].
Improved Approach1 (Approach1 now loads in < 1min).
Added sectioning in Pullbacks2.v
Added Pullbacks3_alt.v
Stripped Pullbacks3.v down to just the approach that works (now compiles in 40s); and added make target for Pullbacks3_alt.v.