use UInt64 for chunk_dimensions

[JonasIsensee]

possibly fixes #451

[codecov[bot]]

Codecov Report

Base: 83.50% // Head: 83.50% // No change to project coverage :thumbsup:

Coverage data is based on head (5580d97) compared to base (5cda06d). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #452 +/- ## ======================================= Coverage 83.50% 83.50% ======================================= Files 30 30 Lines 4135 4135 ======================================= Hits 3453 3453 Misses 682 682 ``` | [Impacted Files](https://codecov.io/gh/JuliaIO/JLD2.jl/pull/452?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/object\_headers.jl](https://codecov.io/gh/JuliaIO/JLD2.jl/pull/452?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL29iamVjdF9oZWFkZXJzLmps) | `38.37% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO)

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[BioTurboNick]

Is there an ETA on the next release? We're running into this a fair bit recently. We can work around it, but it's frustrating my users.