Creating empty HDUs

[jbrinchmann]

Hi,

I need to create FITS files where the first HDU is empty (only containing a header). Given the structure of fits_create_img I did not find a way to do this (please let me know if I overlooked something!). So I extended the code with the following:

function fits_create_empty_hdu(f::FITSFile)

    status = Ref{Cint}(0)
    naxesr = C_NULL
    N = 0
    bitpix = bitpix_from_type(Int16)

    ccall(
         (:ffcrimll, libcfitsio),
         Cint,
         (Ptr{Cvoid}, Cint, Cint, Ptr{Int64}, Ref{Cint}),
         f.ptr,
         bitpix,
         N,
         naxesr,
         status,
     )

    fits_assert_ok(status[])
end

and a small fits_create_img wrapper

function fits_create_img(f::FITSFile, a::Nothing)
    status = fits_create_empty_hdu(f)
end

This appears to work ok but I am not sure whether this is superfluous. If it is useful, I'm happy to create a PR with this, but I must confess I am a tad uncertain about the logic of the runtests.jl so if you have any hints on how to write a test conforming to the code preferences, I'd be happy to hear them!

[thriveth]

For compatibility with other FITS libraries such as astropy, this would be a really nice feature to have implemented in FITSIO.