OutDims should detect backend based on outpath if provided

[felixcremer]

When a path is specified the output should always written to that path and not just kept in memory. I think, that this needs to be changed here, but it might also be changed before in the selection of the oc.desc.backend.

https://github.com/JuliaDataCubes/YAXArrays.jl/blob/eb75dfc2ed163e0bf31018061ed0534d6bd99404/src/DAT/DAT.jl#L803