Once you've called this function on a header unit, subsequent fh_set*() calls will only affect the primary header unit. This has no effect on basic FITS, or if `hu' is already an extension.
`name' should be either "COMMENT" or "HISTORY" and value is the rest of the line (upto 72 characters.)
Use this to set float or `REAL' values. Your value must be of type `double' and will be inserted in the FITS card as a number with a decimal point somewhere, and possibly an exponent (as determined by the instruction to printf). `prec' is a precision value, which should reflect the number of significant digits in `value'.
When printing a year, with a century, prec should be at least 4 to ensure that the value is not printed with an exponent.