Python 0.5.2

Fixes

• Iterating over ts.variants() could cause a segfault in tree sequences
  with large numbers of alleles or very long alleles
  (@jeromekelleher, #2437, #2429).

• Various circular references fixed, lowering peak memory usage
  (@jeromekelleher, #2424, #2423, #2427).

• Fix bugs in VCF output when there isn't a 1-1 mapping between individuals
  and sample nodes (@jeromekelleher, #2442, #2257,
  #2446, #2448).

Performance improvements

• TreeSequence.site position search performance greatly improved, with much lower
  memory overhead (@jeromekelleher, #2424).

• TreeSequence.samples time/population search performance greatly improved, with
  much lower memory overhead (@jeromekelleher, #2424, #1916).

• The timeasc and timedesc orders for Tree.nodes have much
  improved performance and lower memory overhead
  (@jeromekelleher, #2424, #2423).

Features

• Variant objects now have a .num_missing attribute and .counts() and
  .frequencies methods (@hyanwong, #2390 #2393).

• Add the Tree.num_lineages(t) method to return the number of lineages present
  at time t in the tree (@jeromekelleher, #386, #2422)

• Efficient array access to table data now provided via attributes like
  TreeSequence.nodes_time, etc (@jeromekelleher, #2424).

Breaking Changes

• Previously, accessing (e.g.) tables.edges returned a different instance of
  EdgeTable each time. This has been changed to return the same instance
  for the lifetime of a given TableCollection instance. This is technically
  a breaking change, although it's difficult to see how code would depend
  on the property that (e.g.) tables.edges is not tables.edges.
  (@jeromekelleher, #2441, #2080).