Require same datatype for all arrays in multiscales:datasets

[constantinpape]

Fixes #2

I think it is important to include this change rather soon. Right now data with different dtypes across the dimensions is technically valid according to the spec, but will cause issues for several implementations. (It does not work in MoBIE; I am pretty sure the same is true for all tools based on BigDataViewer, and I suspect any tool implemented in a strongly typed language will have issues with this).

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[clbarnes]

I suppose one place where this might not be wanted would be in label datasaets where the bottom level is just a voxel array and higher levels are variable length as labels get collapsed together? Storing that bottom level as variable length could potentially be costly.