Add new date format pattern to DCDate

[kshepherd]

Description

This small change adds an additional date format to DCDate for parsing purposes: yyyy-MM-dd'T'HH:mm:ss'.000'
This date format is used by some APIs and is useful to have in our list of formats to support when parsing date strings.

Instructions for Reviewers

Please add a more detailed description of the changes made by your PR. At a minimum, providing a bulleted list of changes in your PR is helpful to reviewers.

List of changes in this PR:

• New pattern added to DCDate, and included in this class's methods where all patterns are tried or added in succession
• New tests added to DCDateTest to ensure date parsing is successful for the new format

• My PR is small in size (e.g. less than 1,000 lines of code, not including comments & integration tests). Exceptions may be made if previously agreed upon.
• My PR passes Checkstyle validation based on the Code Style Guide.
• My PR passes all tests and includes new/updated Unit or Integration Tests based on the Code Testing Guide.

[nwoodward]

The code changes make complete sense, but is there a way to test them? It appears I can use full ISO dates with milliseconds in date fields now, but I'm guessing some functionality is missing.

[kshepherd]

@nwoodward I added the patterns to DCDateTest (it is one of the files changed by this PR), so that is my test or "proof". As to where this new format / pattern is actually used in DSpace -- it isn't yet, I just wanted to get this small change included now so that future work which does require milliseconds is able to be merged more easily.

[tdonohue]

👍 Thanks @kshepherd . This seems harmless and has tests. Merging immediately.