import or qualify namespace for desc and formalArgs

[dylanhmorris]

From R CMD check:

flag_ww_outliers: no visible global function definition for ‘desc’
preprocess_ww_data: no visible global function definition for ‘desc’
wwinference: no visible global function definition for ‘formalArgs’
Undefined global functions or variables:
  desc formalArgs
Consider adding
  importFrom("methods", "formalArgs")