to_upper - Convert string to upper case

INTERFACE:

 function to_upper(str)
  =========================================================================================

DESCRIPTION:

Convert character string to upper case. Use achar and iachar intrinsics to ensure use of ascii collating sequence.

INPUT PARAMETERS:

   character(len=*), intent(in) :: str ! String to convert to upper case

RETURN VALUE:

   character(len=len(str))      :: to_upper

LOCAL VARIABLES:

   integer :: i                ! Index
   integer :: aseq             ! ascii collating sequence
   character(len=1) :: ctmp    ! Character temporary