NaNs Break metadata table

[jyeatman]

When NaNs are present in afq.metadata, afqbrowser-assemble will leave these cells empty in the .csv file. This causes AFQ-Broswer to either zero pad or to drop those columns. For example, a column with all nans gets dropped making the table and the header not align. Details of the issue aren't important, but we need to find some elegant way to hand nans @arokem @akeshavan @richford

[jyeatman]

@arokem @richford @akeshavan I'm running into this again - There are some subjects with NaN values. afqbrowser-assemble writes these entries out as empty cells rather than keeping the nan and then AFQ-Browser replaces the empty spots with zeros. I thought we handle NaN values? Is there an easy fix?