Use 'a' mode for saving arrays to hdf. Fixes #2642

[Xarthisius]

PR Summary

When saving an array to hdf5 file open the file read/write if it exists, create it otherwise.

Note

Without this PR documentation build is failing.

[neutrinoceros]

note: this collides with #2623, could you please use the keyword syntax to specify the mode ?

Also, evidently this change fixes the issue in the specific case you reported it, but I'm worried that it would create a bug in an other situation where the user specifically wants to overwrite the file.
One way around this would be to expose the "mode" argument in the write_hdf5 function, and to settle for either "w" or "a" as a default. I personally think defaulting to "w" is cleaner but as long as it's documented in the docstring I don't mind if you prefer going the other way :)

[neutrinoceros]

After consideration, this is actually the correct fix since it can be seen as a back port from unyt. Approved !

[Xarthisius]

note: this collides with #2623, could you please use the keyword syntax to specify the mode ?

added.

Also, evidently this change fixes the issue in the specific case you reported it, but I'm worried that it would create a bug in an other situation where the user specifically wants to overwrite the file.
One way around this would be to expose the "mode" argument in the write_hdf5 function, and to settle for either "w" or "a" as a default. I personally think defaulting to "w" is cleaner but as long as it's documented in the docstring I don't mind if you prefer going the other way :)

Clobbering a file via "w" mode appeared here due to #2594 . It changes the behavior of .write_hdf5 without any notice or change to the docstring. I'm merely restoring the API that was accidentally altered. Additionally, it wasn't possible to change the mode before #2594 either.

I'm not opposing changing the default behavior, nor exposing mode as kwargs, but that's not the point of this PR.

[neutrinoceros]

I'm not opposing changing the default behavior, nor exposing mode as kwargs, but that's not the point of this PR.

My mistake ! Indeed my suggestions would belong in another PR, and I'm not convinced it's worth it now.