USAboundaries 0.3.1

New vignette demonstrating the package's functionality (#40).
Additions and clarifications to documentation following @AndySouth's suggestions for JOSS peer review (#38).
us_cities() now returns an sf object rather than a data frame (#36).
us_cities() gains a states argument to match other functions in the package (#35).
Citation to JOSS paper.

USAboundaries 0.4.0

Update all data files to use current version of sf package.
Update Census Bureau data from 2016 to 2020.
Remove the us_boundaries() function was a needless wrapper around other functions.
Prompt user to install data package rather than installing it for them.

Moved most data to USAboundariesData. This improves loading time and permits more frequent updates to the user-facing package.
Added state plane projections table and functions (@jfbratt). Now users can get an appropriate projection for a state.
Converted all boundary objects to sf objects.
Updated all contemporary census boundaries from the 2014 to the 2016 versions.
Added zipcode tabulation area centroids.
Added historical city populations compiled by Erik Steiner at CESTA/Stanford University.

Added contemporary boundaries for states, counties, and congressional districts.
Import many fewer packages. The us_boundaries() function no longer has an option to return a fortified data frame. It is assumed that users will convert the SpatialPolygonsDataFrame objects to whatever format they need.
High resolution data is now available in the USAboundariesData package.