change posterior access interface

[tpapp]

Major changes:

• remove position_matrix as a function (it wasn't exported, so no breaking change)
• document the position_matrix field
• use MCMCDiagnosticTools, remove MCMCDiagnostics
• update sample correctness tests accordingly
• add utility functions for posterior matrices

Incidental changes:

• add test-specific dependencies
• minor doc changes, reorganize repeated docstrings
• put export statements in relevant files
• depend on Julia 1.6
• use concise NamedTuple syntax