%N rdhdd %F $MIR/src/subs/headio.c
%D Read a double precision-valued header variable.
%P mjs
%: header-i/o
%B
FORTRAN call sequence:
        subroutine rdhdd(tno,keyword,value,default)
        integer tno
        character keyword*(*)
        double precision value,default

  Read a double precision valued header variable.

  Input:
    tno         The file handle of the data set.
    keyword     The name of the header variable.
    default     The default value to return, if the header variable
                is not found.
  Output:
    value       The value of the header variable. This will be the default
                value, if the variable is missing from the header.      
