Change allow_symbolic to default to true

From the longer conversation. The reason for not defaulting to it before was a scare that the eliminated expression may be zero. This was found before, leading to NaNs in the resulting system evaluations. However, such zeros would also be problematic to the solver as well, since leaving the uneliminated term in leads to the structural jacobian not matching the true jacobian, which can make the structural DAE index different from the true index. Because of this phenomena, it is no less safe to eliminate the extra variables. But it can lead to some numerical improvements. For example, `p * y ~ p * z` is trivial at `p = 0`, but `y ~ z` is not, and therefore eliminating the `p` is more numerically robust.

Author: ChrisRackauckas

src/systems/systems.jl

@@ -22,12 +22,12 @@ topological sort of the observed equations in `sys`.
 
 ### Optional Keyword Arguments:
 + When `simplify=true`, the `simplify` function will be applied during the tearing process.
-+ `allow_symbolic=false`, `allow_parameter=true`, and `conservative=false` limit the coefficient types during tearing. In particular, `conservative=true` limits tearing to only solve for trivial linear systems where the coefficient has the absolute value of ``1``.
++ `allow_symbolic=true`, `allow_parameter=true`, and `conservative=false` limit the coefficient types during tearing. In particular, `conservative=true` limits tearing to only solve for trivial linear systems where the coefficient has the absolute value of ``1``.
 + `fully_determined=true` controls whether or not an error will be thrown if the number of equations don't match the number of inputs, outputs, and equations.
 """
 function structural_simplify(
         sys::AbstractSystem, io = nothing; additional_passes = [], simplify = false, split = true,
-        allow_symbolic = false, allow_parameter = true, conservative = false, fully_determined = true,
+        allow_symbolic = true, allow_parameter = true, conservative = false, fully_determined = true,
         kwargs...)
     isscheduled(sys) && throw(RepeatedStructuralSimplificationError())
     newsys′ = __structural_simplify(sys, io; simplify,