statnipokladna 0.5.7

Bug fixes

• adapt to new URL scheme for data exports at statnipokladna.cz, fixes #74
• simple codelists with no *_date columns now work in sp_add_codelist(), fixes #66

Minor improvements

• sp_add_codelist() returns a message if joining by multiple columns
• sp_add_codelist() gains a by parameter

Documentation updates

• fixed all outdated links to statnipokladna.cz
• nicer vignette index entries
• vignette and functions now document working with multiple time periods, which differs between datasets/tables
• several clarifications on how to use codelists