Simulation through fixed pedigree

New features

• Add the FixedPedigree ancestry model and various infrastructure for importing pedigree information into msprime.

Bug fixes:

• Fix rare assertion trip in the single sweep model caused by numerical jitter. (#1966, #2038, @jeromekelleher, @molpopgen)

• Fix edge case in Demography.from_old_style() (#2047, #2048, @grahamgower)

Maintenance:

• Documentation improvements (#2054, #2033, #2011 @petrelharp, @gregorgorjanc)