registerInputProcessor() registers an input processor which will be used by
the test recorder. The input processor function should take one parameter,
value, and return a string of R code which returns the desired value.
getInputProcessors() returns a named list of all registered input processor
functions.
registerInputProcessor(inputType, processor)
getInputProcessors()The name of an input type, for example,
"mypkg.numberinput".
An input processor function.