Store odML type in properties for one-to-one mapping

In G-Node/nixpy#313, the `odml_type` was added to `Property` objects to keep track of the original type of odML objects imported into NIX(Py). For consistency and compatibility, NIX should probably support these too, unless we're planning on dropping these types from odML.

No file, test, or entry point is named. Start by tracing how NIX Property objects are serialized and how imported odML types are represented, then compare the behavior described in G-Node/nixpy#313. Done means NIX preserves the odML type needed for one-to-one mapping and compatibility.

Written by the indexing model from the issue text.