Add tspan to *System

[YingboMa]

No description provided.

[isaacsas]

Any chance to support a tspan given in terms of symbolic parameters?

[YingboMa]

Could you give me an example?

[isaacsas]

Maybe something like

@parameters T1 = 0.0 T2 = 1.0
tspan = (T1, T2)
@named sys = ODESystem(eqs, t, [x], ps; defaults = [default_u0; default_p], tspan)
...

# uses the default values or a user parameter map to set the actual numeric tspan
# via varmap_to_vars
prob = ODEProblem(sys)  

The idea would be to keep allowing as much of the system to be specified symbolically as possible by allowing parameters in tspan.

I've thought that it might be useful to do something similar with letting events store symbolic tstop parameters (or maybe that makes more sense in a system), since right now discrete events require users to still manually submit a list of numeric tstops to solve.

[YingboMa]

Yeah, it should be pretty straightforward to implement. Do you want to make a PR?