Absolute time in merge rec

[jchutrue]

This issue is more relevant for the data format of input files to jagular but is documented here for bookkeeping purposes. A jagular file map will extract the first and last timestamps of the passed-in merged rec files, but this is a problem when the files span multiple days, as the timestamps recorded in .rec files restart every time a new rec file is opened. A solution is to store timestamps as 64-bit absolute time in merged rec files so that the timestamps are unambiguous.

[jchutrue]

Note: Reader classes will have to be updated accordingly to handle larger size timestamps.

[eackermann]

We absolutely need to go back to 64-bit timestamps! Let's not keep shooting ourselves in the foot.

[jchutrue]

Timestamps will be signed 64-bit integers with nanosecond resolution, referenced to the Unix epoch time

[eackermann]

For future reference, the decision to use SIGNED integers was made to be consistent with e.g., pandas, and this gives us a range (using nanosecond resolution), of approximately 1677 -- 2262, long after both Josh and I have [hopefully] graduated.