DPath: xs:dateTime fractional second precision lost

Note: This issue is in the dpath-with-serialization branch, not the current 0.15.0

In test xsDateTime_constructor_03, a string is converted to an xs:dateTime with the xs:dateTime constructor function. The string includes microsecond precision (6 digits), but the parsed result rounds to the nearest 3rd fractional digit.

[error] Test edu.illinois.ncsa.daffodil.section23.dfdl_expressions.TestDFDLExpressions.test_xsDateTime_constructor_03 failed: java.lang.Exception:
[error] Comparison failed.
[error] Expected
[error] <dateTime04><string>1998-03-04T12:30:34.123456+00:00</string><dateTime>1998-03-04T12:30:34.123456+00:00</dateTime></dateTime04>
[error] Actual
[error] <dateTime04><string>1998-03-04T12:30:34.123456+00:00</string><dateTime>1998-03-04T12:30:34.123000+00:00</dateTime></dateTime04>

This test is defined in the following file:
daffodil-test/src/test/resources/edu/illinois/ncsa/daffodil/section23/dfdl_functions/Functions.tdml