allow parameter_list objects to be passed to param

[kylebaron]

See discussion in #1075 : a user tried to do this in code and it didn't work; no reason why it shouldn't work.

This PR enables the user to pass a parameter_list object to update parameters in a model object via param()

library(dplyr)
library(mrgsolve)

mod <- house()

mod %>% param(param(CL = 5)) %>% param()

[kyleam]

Makes sense to me.

Confirmed that the new test fails when the code change is reverted.