Update L2 map obs_date to be type CDF_TIME_TT2000

[greglucas]

obs_date should be the same data type as epoch.

Epoch definition:

imap_processing/imap_processing/cdf/config/imap_constant_attrs.yaml

Lines 6 to 22 in 804b7f3

	epoch:
	CATDESC: Time, number of nanoseconds since J2000 with leap seconds included
	FIELDNAM: epoch
	LABLAXIS: epoch
	FILLVAL: -9223372036854775808
	FORMAT: " " # Supposedly not required, fails in xarray_to_cdf
	VALIDMIN: 315576066184000000 # 2010-01-01T00:00:00 mission start (APL 0 epoch)
	VALIDMAX: 3155716869184000000 # 2100-01-01T00:00:00 mission end
	UNITS: ns
	VAR_TYPE: support_data
	SCALETYP: linear
	MONOTON: INCREASE
	TIME_BASE: J2000
	TIME_SCALE: Terrestrial Time
	REFERENCE_POSITION: Rotating Earth Geoid
	RESOLUTION: ' '
	DICT_KEY: "SPASE>Support>SupportQuantity:Temporal"

obs_date definition:

imap_processing/imap_processing/cdf/config/imap_enamaps_l2-common_variable_attrs.yaml

Lines 314 to 324 in 804b7f3

	obs_date: &obs_date
	<<: *default_int64
	datatype: int64
	CATDESC: Mean collection date of data in a pixel
	FIELDNAM: J2000 Nanoseconds
	UNITS: ns
	DEPEND_0: epoch
	VAR_TYPE: support_data
	LABLAXIS: epoch
	DISPLAY_TYPE: no_plot
	DICT_KEY: SPASE>TemporalDescription:TimeSpan,Qualifier:Directional,CoordinateSystemName:HAE,CoordinateRepresentation:Spherical

Likely just copy the epoch definition over to obs_date here is the easiest path forward.