ISCE3 DateTime objects cannot parse strings that specify UTC via "Z" ending

Attempting to run a job, I encountered an error when attempting to parse a string:

ValueError: Error in file /src/cxx/isce3/core/DateTime.cpp, line 413, function void isce3::core::DateTime::strptime(std::string): ISO-8601 incompatible or bad value for datetime string!

The offending string was a UTC datetime that was in the following style:
'2026-01-01T00:00:00.000000Z'

Discussing this with @bhawkins, he pointed out that of the following three statements, only the third succeeds:

from isce3.core import DateTime
DateTime('2026-01-01T00:00:00.000000Z')
DateTime('2026-01-01T00:00:00.00Z')
DateTime('2026-01-01T00:00:00.000000')

He also pointed out that isce3 uses sscanf under the hood for parsing strings to DateTime objects.

The evidence points to isce3.core.DateTime objects being unable to parse datetimes that end with "Z", which may be confusing to users.

Start in src/cxx/isce3/core/DateTime.cpp at the strptime implementation around line 413, and reproduce the three DateTime constructor examples from the issue. Check the existing parsing behavior for fractional seconds and a trailing Z; done means UTC strings ending in Z parse successfully without breaking the formats that already work.