[Nexus] HDF error log

[Le Goc Yannick]

Hi,

When writing quite big datasets with Nexus HDF5, I realized that 
sometimes the gzip compression wasn't applied.
I wanted to go into details and try to understand why the library was 
behaving like this.
As Nexus is a thin layer on top of HDF, I supposed it was due to the HDF 
library.
I tried to log the HDF5 errors, but I found no way to do it with Nexus. 
Then I introduced HDF5 calls in my code like this:

herr_t my_hdf5_error_handler(hid_t err_stack, void *unused) {
   H5Eprint1(stderr);
    return 0;
}

NXstatus ret = NXopen(const_cast<char *>(m_fileName.c_str()), 
NXACC_CREATE5, &file_id);

H5Eset_auto(H5E_DEFAULT, my_hdf5_error_handler, NULL);

This gave me the error message as expected, but I also had the following 
errors :
"H5Aopen_by_name(): can't open attribute" for many attributes called 
"napimount" and some "signal" attributes.

As my file seems correctly written, I have the impression that this is a 
bad use of HDF5. The attribute existence should be tested before its 
request.

My wishes:
- fix the "napimount" and "signal" attribute error as they do not seem 
to be real errors
- have the possibility to log HDF errors, maybe by adding a function 
NXsethdferrorhandler to the Nexus API that unifies HDF5/HDF4 error logs. 
Or at least, add a small section in the documentation on error handling 
including HDF error handling (maybe with the above code example).

I'm using Nexus version 4.3.0

Regards,

Yannick