Move withr to suggests

[Nic-Chr]

See R CMD check note: https://cran.r-project.org/web/checks/check_results_phsmethods.html

Moving withr from Description file Imports to Suggests should fix it.

Check Details
Version: 1.1.0
Check: dependencies in R code
Result: NOTE
Namespace in Imports field not imported from: ‘withr’
All declared Imports should be used.
Flavors: r-devel-linux-x86_64-fedora-clang, r-devel-linux-x86_64-fedora-gcc