Overhaul stem family of plot functions. * scripts/plot/private/__stem__.m: Use low-level plotting fcn __line__ for performance. Add property/listener on 'basevalue' for baseline object. Simplify and correct input option processing so that it actually does the right thing. * scripts/plot/stem.m: Add list of "stem series" properties to docstring. Add titles to %!demos. Add %!error tests for input validation. * scripts/plot/stem3.m: Cross-reference "stem series" to stem documentation. Accept property/value pair inputs. Add %!error tests for input validation.

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