Retain all properties when writing to a file

[ValentinKaisermayer]

ModelingToolkit.jl/src/systems/abstractsystem.jl

Line 511 in 198e6e9

function toexpr(sys::AbstractSystem)

For the ODESystem the generated file does not hold:

• connection_type
• controls
• observed

[ValentinKaisermayer]

Not sure if #1127 already covers this.

[ChrisRackauckas]

It should cover a good amount of it at least.