add general euler-angle functions,
	euler2t t2euler euler2quat quat2euler
	meuler2t t2meuler meuler2quat quat2meuler
	The "meuler" forms take axis-order specs in Maya style,
	but the angles are accepted/returned always in X,Y,Z order,
	so meuler2t( "zxy", 17,35,-112 ) = rotZ(-112) * rotX(17) * rotY(35).
	"zxy" is the same as the virdir axis order.

	The "euler" forms take axis-order specs in the reverse of Maya style,
	and expect/return angles in the same order as the axis-order spec,
	e.g. euler2t( "yxz", 35,17,-112 ) = meuler2t( "zxy", 17,35,-112 ).

   add stats()
	Accept a sequence of numbers, and accumulate their
	mean and variance.
	   ($svar, $N, $mean, $SDev) = &stats(val1, val2, ...)
	To accumulate more values, pass the first result ($svar)
	   ($svar, $N, $mean, $SDev) = &stats($svar, valN, valN+1, valN+2, ...)