Add write-support for compound data type

[halset]

Trying to write a HDF5-file with java and got stuck by not being able to encode a compound data type. Seems like DataType.fromObject are not able to detect and return a CompoundDataType.

[jamesmudd]

That's correct writing compound datasets is not supported at the moment. Think it will be relatively complex to implement so probably won't be addressed soon.

[hiSandog]

Compound writing probably needs an explicit schema/builder rather than relying only on DataType.fromObject, because Java reflection does not define HDF5 field order, offsets, padding, or unsigned/nested types reliably. A first slice could support a declared list of named scalar members and arrays, then add nested compounds once layout round-trips are stable. Compatibility tests should write with jHDF and read with h5py/HDF5, covering alignment and variable-length strings.