is_digit Function

   elemental function is_digit(self)
   !< Return true if all characters in the string are digits.
   !<
   !<```fortran
   !< type(string) :: astring
   !< logical      :: test_passed(2)
   !< astring = '   -1212112.3 '
   !< test_passed(1) = astring%is_digit().eqv..false.
   !< astring = '12121123'
   !< test_passed(2) = astring%is_digit().eqv..true.
   !< print '(L1)', all(test_passed)
   !<```
   !=> T <<<
   class(string), intent(in) :: self     !< The string.
   logical                   :: is_digit !< Result of the test.
   integer                   :: c        !< Character counter.

   is_digit = .false.
   if (allocated(self%raw)) then
      do c=1, len(self%raw)
         select case (self%raw(c:c))
         case ('0':'9')
            is_digit = .true.
         case default
            is_digit = .false.
            exit
         end select
      enddo
   endif
   endfunction is_digit