Assemble AD2CP timestamp with nanosecond precision

[leewujung]

Previously when assembling timestamp for AD2CP files, the precision was not explicitly set and defaulted to us. This causes issues with encoding times in xarray: When the time is encoded with us, coding.times.encode_cf_datetime returns negative values, which are then converted to NaT in coding.times.decode_cf_datetime and causes whole bunch tests failing seen in here.

This PR addresses this issue by explicitly set precision to ns when assembling timestamp in parse_ad2cp.py::Ad2cpDataPacket.timestamp.

The encode-decode code block:

echopype/echopype/utils/coding.py

Lines 81 to 96 in f64e8c3

	else:
	# fmt: off
	encoded_data, _, _ = coding.times.encode_cf_datetime(
	da, **{
	"units": DEFAULT_TIME_ENCODING["units"],
	"calendar": DEFAULT_TIME_ENCODING["calendar"],
	}
	)
	# fmt: on
	return coding.times.decode_cf_datetime(
	encoded_data,
	**{
	"units": DEFAULT_TIME_ENCODING["units"],
	"calendar": DEFAULT_TIME_ENCODING["calendar"],
	},
	)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.66%. Comparing base (9f56124) to head (ec15e95).
Report is 166 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1436      +/-   ##
==========================================
- Coverage   83.52%   80.66%   -2.86%     
==========================================
  Files          64       19      -45     
  Lines        5686     3108    -2578     
==========================================
- Hits         4749     2507    -2242     
+ Misses        937      601     -336     

Flag	Coverage Δ
unittests	80.66% <ø> (-2.86%)	⬇️

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