[RTM] move missing values to the beginning of the confounds file

[chrisgorgo]

It's more intuitive in context of using those for nuissance regression.

[effigies]

This is beyond my pandas experience. I'm willing to take your word that this does what you say, though.

[chrisgorgo]

Let me add a changelog entry

[effigies]

Resolved conflicts in changelog. I can't cancel the resulting build for you, but there's no need for it to run to merge this.