feat: Square, symmetric, positive semi-definite LinearOperator.

[daniel-dodd]

All operators are assumed to be positive semi-definite, except for the lower and upper triangular linear operators. In future, we will abstract this property out, and allow for non-square matrices.

Do we do this something like this?

class SquareLinearOperator(LinearOperator):
    ...

class SymmetricLinearOperator(SquareLinearOperator):
    ...

class PositiveSemiDefiniteLinearOperator(SymmetricLinearOperator):
    ...

class PositiveDefiniteLinearOperator(PositiveSemiDefiniteLinearOperator):
    ...

Or do we define these as properties on the linear operator?

Resolving this would allow to move beyond square operators.