v0.6

• Merge pull request #17 from ihmpdcc/pr-16
• Pull request from @schwager-hsph merged along with another commit.

v0.5

• Merge pull request #15 from ihmpdcc/study_linkage_logic_error
• fix: study linkage logic error

v0.4

• Added convenience methods for several classes to easy retrieval
  of linked objects. Contributed by @schwager-hsph.

v0.2

• Merged pull request providing search() implementation for each
  node type.