Change refTime to refTime_offset in ChildSpec

[tclune]

I was mislead by ESMF documentation, and we need the actual refTime for user clocks to be something more realistic than 900s AD.

Needs the following changes:

• Change refTime as an ESMF_Time to refTime_offset as an ESMF_TimeInterval
• Change the schema for the user to specify refTime_offset instead of refTime
• Modify the analogous component in OuterMetaComponent refTime to be refTime_offset
• Modify OuterMetComponent/add_child_by_spec.F90 refTime ==> refTime_offset
• In OuterMetaComponent/initialize_set_clock.F90: user_refTime = outer_refTime + refTime_offset