ctor_R16P Function

   function ctor_R16P(str, knd, pref, error) result(n)
   !< Convert string to real.
   !<
   !<```fortran
   !< use penf
   !< print FR16P, cton(str='-1.0', knd=1._R16P)
   !<```
   !=> -0.100000000000000000000000000000000E+0001 <<<
   character(*),           intent(in)  :: str   !< String containing input number.
   real(R16P),             intent(in)  :: knd   !< Number kind.
   character(*), optional, intent(in)  :: pref  !< Prefixing string.
   integer(I4P), optional, intent(out) :: error !< Error trapping flag: 0 no errors, >0 error occurs.
   real(R16P)                          :: n     !< Number returned.
   integer(I4P)                        :: err   !< Error trapping flag: 0 no errors, >0 error occurs.
   character(len=:), allocatable       :: prefd !< Prefixing string.

   read(str, *, iostat=err) n ! Casting of str to n.
   if (err/=0) then
     prefd = '' ; if (present(pref)) prefd = pref
     write(stderr, '(A,I1,A)') prefd//' Error: conversion of string "'//str//'" to real failed! real(', kind(knd), ')'
   endif
   if (present(error)) error = err
   endfunction ctor_R16P