mix of changes: :/
sexagesimal:
	Angle types reorganized into Angle/FmtAngle.
	Time type added.
	Doc.go updated accordingly.
	Various test files updated to use these new types.
interp:
	Added Slice function.
deltat:
	Added table 10.A
	Added example interpolating from table.
greenwich:
	Fix MeanSidereal to return non-negative.