* add lots of static assertions making it very explicit when all these ops
are supposed to work:
** all ops require the rhs vector to go in the right direction
** all ops already require that the lhs and rhs are of the same kind
(matrix vs vector) otherwise we'd have to do complex work
** multiplicative ops (introduced Kibeom's patch) are restricted to arrays, if only because for matrices they could be ambiguous.
* add a new test, vectorwiseop.cpp.
* these compound-assign operators used to be implemented with for loops:
for(Index j=0; j<subVectors(); ++j)
subVector(j).array() += other.derived().array();
This didn't seem to be needed; replaced by using expressions like operator+ and operator- did.
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