Add 'linear' attribute to StationaryModel and InstationaryModel

[pmli]

(Part of the changes from #652)

Since rhs and mass are asserted to be linear, it should be enough to add self.linear = operator.linear.

[sdrave]

There is also the outputs dict. Is a Model linear when Operator is linear but one of the outputs Operators is nonlinear? I am not completely sure what the right definition would be, but I would tend to require the outputs to be linear as well.

[pmli]

Right, for the input-output system to be linear, the output should also be linear (although a linear input-output system with a nonlinear operator and output could be constructed...).