Non-positive Ts for continuous types

[albheim]

Noticed that some methods in pid_design.jl still used non-positive Ts to denote continuous system. Seems rather harmless since default if no Ts is supplied it returns continuous and if a valid Ts is supplied (Ts>0) it returns discrete. Just thought I would put it here anyway.

https://github.com/JuliaControl/ControlSystems.jl/blob/eb0db1ac1a6285dd142b86052d4b241672617c3a/src/pid_design.jl#L157-L163

[olof3]

I think it would be good to fix this. For consistency it should also be Ts instead of h, which we concluded was the overall better choice. Personally, I am used to seeing h in this context, but it makes it painfully inconsistent with the rest of ControlSystems.jl. I guess it would also be slightly better to use nothing rather than magic numbers <0, I don't think it would make sense to have Ts = Continuous().

[albheim]

Yeah, I can have a look at it in #509