`time` index needs validation across channels

[simmsa]

In one .mcap file there are multiple channels. Right now each channel's time is converted independently using the system_time column. This works, but it does not validate that time makes sense for all channels.

Recommended action in to check if start time for each channel is within some tolerance (1 second) and the same for the end time. May need to exclude channels that do not have a consistent $\delta t$, which is allowed for HMI channels.