generate obs_fun, obs_dfdx, obs_dfdp autmatically if only choosing component of state variable

[jonaspleyer]

Assuming that the state variable is x_0 ... x_n
If we pick the observable to be eg. g(x) = x_i for a specific i then we can automatically generate the needed observable functions.
The same can be said for if multiple values are chosen such as g(x) = [x_0, x_2, x_100]