Close open temporary file descriptor when reading par file from HDF5 HeterodynedData object

In !191 (merged) I tried to solve the issue of too many files being open when generating the summary pages, but apparently it didn't solve the problem. I think I might have tracked down the issue. When reading in a HeterodynedData object from a HDF5 file, the par file information is stored in a string, but to get it back into the new HeterodynedData object I need to temporarily store it in a file. To do this I used mkstemp to create a temporary file. But, I was ignoring the first value that returns, which is a file descriptor pointing to the open file. I was subsequently re-opening and close the file, but not closing that original file descriptor. I think this might have left an open file hanging about for each HeteodynedData object read in.

This MR fixes this, bu actually returning the file descriptor and manually closing it.