Added element X to the elements list in order to support unknown species

[espenfl]

Can be useful in parsing scenarios when the element is now known, or in cases where one does not explicitly handle atoms (electrons, orbital calculations etc.). It is then also possible to detect element X down the line and request user for additional information or do automatic corrections, if needed.

[codecov-io]

Codecov Report

Merging #1613 into develop will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1613      +/-   ##
===========================================
+ Coverage    57.14%   57.17%   +0.02%     
===========================================
  Files          273      273              
  Lines        33773    33773              
===========================================
+ Hits         19300    19309       +9     
+ Misses       14473    14464       -9

Impacted Files	Coverage Δ
aiida/orm/data/structure.py	78.21% <ø> (ø)	⬆️
aiida/common/constants.py	100% <ø> (ø)	⬆️
aiida/backends/djsite/db/models.py	75.97% <0%> (+0.88%)	⬆️
aiida/backends/djsite/globalsettings.py	86.84% <0%> (+5.26%)	⬆️

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[DropD]

@giovannipizzi I don't see a problem with this, but I would like your approval.

[giovannipizzi]

Indeed it might be sufficient.

Could you also update the docstrings in the same file (for some reason the sentence stops...), and
https://github.com/espenfl/aiida_core/blob/e5b6ab8a25fbce5462be03f5a700d43675378832/aiida/orm/data/structure.py#L229

Great if you could also check the current tests for StructureData, and try to duplicate all of them to use X and check that everything still works

[espenfl]

Most of the tests duplicated (including ASE test). Tests are successful.

[giovannipizzi]

Thanks!