Ignore nans in least_squares_fit(). Avoid using days as datetime64 unit.

Conversion between datetime64 instances which do not have times and
those which do (ie those based on day or larger units, and those based
on hours or smaller units) is troublesome because of timezone
issues. Avoid using day as unit when hours can be used instead.
Avoid using day as unit for datetime64/timedelta64 due to timezone issues.
Avoid using day as unit for datetime64/timedelta64 due to timezone issues.
Avoid using day as unit for datetime64/timedelta64 due to timezone issues.
Accept MagQDC or aligned version in constructing AuroraWatchActivity.