Use the NPZ object serialization of Sigima in the Web API #429

Problem

The NPZ format of the Web API is implemented twice: in DataLab (datalab/webapi/serialization.py) and in DataLab-Kernel (datalab_kernel/serialization_npz.py). The copies diverge (non-uniform image coordinates had to be added to both; the Kernel cannot transfer metadata holding NumPy arrays nested in lists).

Proposal

Sigima now provides the implementation, taken from DataLab (DataLab-Platform/Sigima#90, sigima.io.serialize_object_to_npz / deserialize_object_from_npz). DataLab uses it and keeps only what is specific to the API (object_to_metadata); the format and its behavior are unchanged.