[x] Have you successfully run devtools::check() locally?
[x] Have you updated or added function (and vignette if applicable) documentation? Did you update the 'man' and 'NAMESPACE' files with devtools::document()?
[x] Have you added in tests for the changes included in the PR?
[x] Do the changes follow the ihmeuw-demographicscode style?
[ ] Do the changes need to be immediately included in a new build of docker-base or docker-internal? If so follow directions in those repositories to rebuild and redeploy the images.
[ ] Do the changes require updates to other repositories which use this package? If yes, make the necessary updates in those repos, and consider integration tests for those repositories.
[ ] If this is a private package did you use Jenkins to rebuild the internal pkgdown site?
Describe changes
Adds simple assertion that the lower interval must be less than the upper interval
What issues are related
Fixes #42
Checklist
Packages Repositories
ihmeuw-demographicsR packages?devtools::check()locally?devtools::document()?ihmeuw-demographicscode style?docker-baseordocker-internal? If so follow directions in those repositories to rebuild and redeploy the images.