Write merged SD and rec file data directly to individual channels

[jchutrue]

In our current workflow, we take a rec file along with SD card file to create a merged rec file. This step can often be necessary for the following reasons:

1. The merged rec file contains all neural data channels. This is inefficient especially if we don't record from all channels. The same can be said for the DIOs.
2. To get a single channel data out, we go from packed data (sd file) to packed data (merged rec file) to individual channel data. It is arguably more time-consuming to do it this way when we can eliminate the intermediate merged file and go directly from packed data in the sd file to individual channel data.

Finally, the timestamps should be in absolute time to allow for pain-free comparison across days (or even within a single day if there were multiple recordings taken in a day). See #22