EAMxx: allow TimeStamp update with fractional value of seconds

[bartgol]

While an atm step will always land on an integer second, subcycling of atm process can cause the timestamp to be fractional. Hence, we need to be able to update with fractional seconds, keeping track of the fractional part and reset the tally to 0 whenever we're "close" to integer seconds, to avoid carrying around roundoffs.

Branch bartgol/eamxx/enhance-timestamp-update contains some initial work, but the TimeStamp is not completely self-consistent. E.g., seconds_from does not account for the fractional part. To ensure the class is self consistent, we should remove that method. Since it's basically only used in the TimeInterpolation class, which is destined to be removed in favor of DataInterpolation, we can put this issue on hold until the removal of that class.