Switch internal time units to microseconds

[spencerkclark]

📰 Custom Issue

Historically the time units used in nc-time-axis have been "days since 2000-01-01". cftime.num2date and cftime.date2num are exact as of cftime version 1.2.1 if units of microseconds are used, since all arithmetic can be done with integers. Therefore it might be beneficial to switch to using microseconds internally within nc-time-axis. Errors are very small if we do not use microseconds, but we might as well be as exact as possible.