Update AiiDA REST API

[ltalirz]

This adds a few fixes to the AiiDA REST API

• structure endpoint can now return structure data in different formats (.xyz, .cif,...)
• adds server info endpoint
• hide sensitive user information

[ltalirz]

Thanks for approving @giovannipizzi , but this was perhaps a bit fast ( @waychal still wanted to have a look).
I just tried the download feature on my machine:
http://127.0.0.1:5000/api/v2/structures/10a9728f/content/download?format=cif
and it crashed the REST API

 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
python(82523,0x70000f6f9000) malloc: *** mach_vm_map(size=12202731648260771840) failed (error code=3)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
Segmentation fault: 11

[giovannipizzi]

Wow, a segmentation fault? Never seen one with python... Do you manage to understand which call is the cause? Is there a test for this format? (BTW, is this a small or a very big structure?)

[giovannipizzi]

I just tried on my Mac and I don't get errors... probably it's a bug in the version of PyCifRW we use (btw, very old), and maybe happening for big structures? @nmounet and @merkys have been discussing upgrading pycifrw to a more recent version a few days ago, with the issue that PyCifRW changes API every new version... @nmounet, @merkys did you decide which version to upgrade to?

BTW, @waychal : I just realised that you check if the output string you get for the CIF file is exactly the same (https://github.com/aiidateam/aiida_core/blob/0e0a7c55a7db70d644b47930ed7137a8cd3cc84e/aiida/backends/tests/restapi.py#L766) However, as PyCifRW often slightly changes the format of the output (e.g. more spaces, more comments, ...) between one version and another, it would be better to just check if the file is valid even if slightly different. E.g., if there are specific substrings like _atom_site_type_symbol etc.; or by reloading the CIF file and checking, e.g., the number/type of atoms.

[ltalirz]

This discussion will continue in #882

[merkys]

@giovannipizzi: I could replace pycifrw with pycodcif (see my post in #882), however, I need a help to C-compile it on Mac. Also, this would require a migration of CifData, but interface/data structure of pycodcif is less bound to change.