Lsim docstring update

[stepanoslejsek]

The example with u function in the lsim docstring throws an error which suggests a broadcasting. I figured out a solution to this: using u = (x,t)->[t * Ts >= t0] instead of u = (x,t)->(t * Ts >= t0) for a step function at t0.

[mzaffalon]

With v1.5.1 this reports the error correctly:

julia> lsim(model, (x,t)->(t >= 0.5), 2)
ERROR: u must be a vector of size (1,)

and can be closed.