posterior 1.7.0

Bug Fixes

• Fix several minor issues occurring in edge cases.

Enhancements

• Extend and export several functions related to the generalized
  Pareto distribution for use in other packages. (#408)
• Support Pareto PIT functionality via pareto_pit. (#435)
• Allow more warnings to be turned off via option
  posterior.warn_on_merge_chains.
• Improve documentation in many places.
• Improve tests by checking for edge cases.