Record LabVIEW version in metadata

[ageorgou]

• Add an attribute to the lab metadata to hold the LabVIEW version (unless creating from metadata only)
• Update large file signatures to also reflect the changes from #58.

The new attribute is optional, but I wonder whether it's best to always have it and set it to "None" or "N/A" if we don't have any LabVIEW data (e.g. if creating a minimal NWB file from metadata only).

[ageorgou]

I have removed the spec from the signature to avoid spurious differences when we update it. This should be fine, as we can always find which version of the code a signature was generated from through the history.

The downside is that we may accidentally end up creating files that don't contain the spec for the extension at all. I've tried creating a round-trip test that checks that a file can be loaded, but this is trickier than it seems: pynwb keeps the extension namespaces loaded after writing the file, so the test doesn't really mean anything. I haven't found a way to remove them, and reloading the module with importlib.reload breaks all reading (even without any extensions).