package can't be installed because `assertive` is defunct

[dholstius]

Summary

• Package assertive has been removed from CRAN
• Because of this, packages that depend on assertive can't be installed automatically (with, e.g., `devtools::install_github("BAAQMD/geotools"); nstallation will fail
• This package relies on assertive.numbers in a really unnecessary way

[dholstius]

Fixed via 824fbc2