Add support for random access groups

[bensetterholm]

I am trying to write a FITS-IDI file. According to the file standard (see §3.1), the first HDU block must be a "random-groups FITS data set that contains no data." However, this is not an HDU type supported by FITSIO.jl.

When loading the example file from the registered FITS conventions page, FITSIO.jl reads the first block in as an Image HDU. Attempting to read the data leads to an error (CFITSIO has encountered an error. Error code 320: illegal number of dimensions).

If I attempt to save a file, I cannot get NAXIS to set to zero, and if I try to set GCOUNT or PCOUNT values in a FITSHeader object, they seem to disappear upon file write.

Related: #111