Import cell internal resistance into final output data

[darkwraith]

Many cyclers measure internal cell resistance at the beginning or end of a cycle. This data should be included in the exported data. This is more useful at the per cycle "summary" export, but I believe it is also needed in the raw export for troubleshooting, removing/identifying outliers, and ease of plotting.

[rwaldheim]

The internal resistence is already included in the individual channel files (the large files that contain everything the Arbin exports + a few more). We currently only record it once, so I'm not necessarily sure where to put it since there isn't really a "cell summary" file yet. If we start recording it on a cycle basis, I can include it in cycle_facts.