check.fs: Check Vector of Nonexceedance Probabilities
Description
This function checks that a nonexceedance probability (\(F\)) is in the \(0 \le F \le 1\) range.
It does not check that the distribution specified by parameters for \(F = 0\) or \(F = 1\) is valid. End point checking is left to additional internal checks within the functions implementing the distribution. The function is intended for internal use to build a flow of logic throughout the distribution functions. Users are not anticipated to need this function themselves. The check.fs function is separate because of the heavy use of the logic across a myriad of functions in lmomco.