Update data to include all connections

[lparsons]

Use threshold of zero (0) when exporting data to ensure that all connections are included in the graph.

In addition

• defaults were added to the help for export_atlas_scalar function: https://github.com/leiferlab/worm-functional-connectivity/pull/15
• setting of the threshold was documented in the data update process in NPLCAdmindocs

Resolves #120.