[feature] Add a method to compute A_out

[Phionx]

After solving for a(t), we can compute A_out(t) using the input-output relation, as described by eq(2.20) of Ref [1].

[1] https://cpb-us-w2.wpmucdn.com/campuspress.yale.edu/dist/2/3627/files/2020/10/steven_thesis.pdf

I believe this can be done using the quantities framework in analysis/quantities.