Rename util.py to datetimeutil.py

Since in fact it only contains date and time manipulation helpers, which
we're going to further expand.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Remove datetimeutil.format_date function

Nothing uses it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Test and handle case of a system with no inverters specified

For completeness.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Add timezone attribute to systems

Defaults to localtime (at the place smadata2 is running);

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Implement helper for turning datetime objects to POSIX timestamps

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Add day_timestamps() function

This one generates POSIX timestamps for the beginning and end of a
specific date in a specific timezone.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>