Update to general namespace separator

This is the one place where it's used? Thoguh, does this code need to exist? It seems like it's a separate implementation of SII that is a bit buggy, @TorkelE might be best to just replace with SII?

Author: ChrisRackauckas

src/reactionsystem_conversions.jl

@@ -798,7 +798,7 @@ function _symbol_to_var(sys, sym)
     if hasproperty(sys, sym)
         var = getproperty(sys, sym, namespace = false)
     else
-        strs = split(String(sym), "₊")   # need to check if this should be split of not!!!
+        strs = split(String(sym), ModelingToolkit.NAMESPACE_SEPARATOR)   # need to check if this should be split of not!!!
         if length(strs) > 1
             var = getproperty(sys, Symbol(strs[1]), namespace = false)
             for str in view(strs, 2:length(strs))