Fix nested flowsheet time domain construction

[NishantGiridhar]

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Fixes

Summary/Motivation:

In the current implementation of FlowsheetBlock, nested flowsheets with a time domain can only inherit time from the parent flowsheet.

Changes proposed in this PR:

• Moved code to create a new time domain into a private method.
• Create a new time domain for a nested FlowsheetBlock if time_set is provided during construction.
• Added tests.

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