sphere.m: Add input validation of input N. * sphere.m: Check that input N is a real scalar > 0. Add BIST checks for new input validation. Rename output variables "xx,yy,zz" to "x,y,z" to match documentation. Rename internal variables "x,y,z" to "xx,yy,zz". Add %!demo.

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[*.{am,mk}]
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