Add SectionItems.get(mnemonic, default, add=False)

[kinverarity1]

Analogous to dict.get but with extra "add" kwarg.

SectionItems.get should accept strings or HeaderItem objects for the "default" arg. If it is a string, it should be converted to a HeaderItem with the appropriate mnemonic.

Always returns a HeaderItem.

If add is True, the returned value should also be appended to the SectionItems instance.

How do we handle CurveItems? Set the data attribute to all nans, with the appropriate length?