Don't conflict with HasHDF in StructureStorage

[pmrv]

Drops support for legacy storage format.

afaik, the old format of saving StructureStorage to HDF is no longer used and therefore drop it, could this bite you in some way @Leimeroth ?

Depends #375

[Leimeroth]

I think I loaded all jobs that used this and rewrote hdf files using to_hdf() manually at some point, but I will have to check.
Even if there are some problems I guess it would still be possible to load and rewrite old jobs using an older version if you merge this and than switch back to the new version or am I overlooking something?

[coveralls]

Pull Request Test Coverage Report for Build 1282030235

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 69.779%

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Totals
Change from base Build 1279860898:	0.02%
Covered Lines:	11413
Relevant Lines:	16356

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💛 - Coveralls

[pmrv]

I think I loaded all jobs that used this and rewrote hdf files using to_hdf() manually at some point, but I will have to check.
Even if there are some problems I guess it would still be possible to load and rewrite old jobs using an older version if you merge this and than switch back to the new version or am I overlooking something?

Yeah, downgrading pyiron temporarily, rewriting and then upgrading should be no problem.