Make ESMF_FieldGetTimeStamp a documented method

[tclune]

Currently this and the corresponding Set method are undocumented. MAPL3 is likely to need similar functionality, though for a slightly different purpose.

But ... before the interface is made public, I think there should be a discussion on the representation of TimeStamp itself. Currently it is not a derived type but a managed list of integers extracted from an ESMF_Time object. I can imagine that there are circular dependencies that prevent use of ESMF_Time in this context, and/or maybe ESMF_Time is too heavyweight. But a derived type would seem appropriate.

[anntsay]

will need more discussion with Tom C.'s group.

may require rethinking of architecture and see if we could leverage this proposal to add more features if need to do this work.