fix: convert to datetime first in format_epoch to retain millisecond precision

[kyle-cochran]

There is another fix for this I believe. astropy.Time takes an optional precision field. From the docs:

precision : int, optional
    Digits of precision in string representation of time

For example:

test_string = "2024-02-08T19:46:03.597928"
assert Time(test_string, precision=6).strftime("%Y-%m-%dT%H:%M:%S.%f") == test_string #true
assert Time(test_string).strftime("%Y-%m-%dT%H:%M:%S.%f") == test_string #false

Could also add a precision arg to everywhere Time is used, but that might be harder to keep track of in the library. Let me know your preference.

[coveralls]

Pull Request Test Coverage Report for Build 8843274942

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 93.67%

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Totals
Change from base Build 8218439654:	0.03%
Covered Lines:	873
Relevant Lines:	932

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