Improve input constructor implementations

[nanxstats]

We received some feedback from Keaven on not being able to reuse the enroll/failure rate objects in the design object under the new define_*() input constructors and checking.

We should

1. Check why the classes of the rate objects returned by gs_design_ahr() is not preserved. (need a minimal reproducible example from Keaven).

2. Improve the warning message to soften the tune: it's possible to construct the object manually under certain conditions, for example, with certain classes assigned.

[nanxstats]

Proposal from @jdblischak: validate if the input is a valid input, instead of a certain class.

[jdblischak]

Here is a quick reproducible example that produces the warning messages:

library("gsDesign2")

enroll_rate <- define_enroll_rate(
  duration = c(2, 2, 10),
  rate = c(3, 6, 9)
)
class(enroll_rate)
## [1] "enroll_rate" "tbl_df"      "tbl"         "data.frame" 
gs_design_ahr(enroll_rate = enroll_rate)

# remove the class
class(enroll_rate) <- class(enroll_rate)[-1]
class(enroll_rate)
## [1] "tbl_df"     "tbl"        "data.frame"
gs_design_ahr(enroll_rate = enroll_rate)
## Warning messages:
##   1: In check_enroll_rate(enroll_rate) :
##   Please use `define_enroll_rate()` to specify the `enroll_rate` argument.
## We will enforce this requirement from the next version.
## 2: In check_enroll_rate(enroll_rate) :
##   Please use `define_enroll_rate()` to specify the `enroll_rate` argument.
## We will enforce this requirement from the next version.
## 3: In check_enroll_rate(enroll_rate) :
##   Please use `define_enroll_rate()` to specify the `enroll_rate` argument.
## We will enforce this requirement from the next version.

So I plan to update the function check_enroll_rate() to verify the input instead of enforcing the class. If the input is invalid, I will have the error message recommend the user try the helper function define_enroll_rate()

gsDesign2/R/check_arg.R

Lines 109 to 116 in 187edf3

	check_enroll_rate <- function(enroll_rate) {
	if (!("enroll_rate" %in% class(enroll_rate))) {
	msg <- c(
	"Please use `define_enroll_rate()` to specify the `enroll_rate` argument.",
	"We will enforce this requirement from the next version."
	)
	msg <- paste(msg, collapse = "\n")
	warning(msg)