Default is_*_sym to false, update docstrings

- `is_indep_sym`, `is_state_sym` and `is_param_sym` default to false
  for types that don't implement a more specific method

Author: AayushSabharwal

src/interface.jl

@@ -8,7 +8,8 @@ function independent_variables end
 """
 $(TYPEDSIGNATURES)
 
-Check if the given sym is an independent variable in the given system.
+Check if the given sym is an independent variable in the given system. Defaults
+to `false` if not implemented for the given system/container type.
 """
 function is_indep_sym end
 
@@ -29,7 +30,8 @@ function state_sym_to_index end
 """
 $(TYPEDSIGNATURES)
 
-Check if the given sym is a state variable in the given system.
+Check if the given sym is a state variable in the given system. Defaults
+to `false` if not implemented for the given system/container type.
 """
 function is_state_sym end
 
@@ -50,6 +52,7 @@ function param_sym_to_index end
 """
 $(TYPEDSIGNATURES)
 
-Check if the given sym is a parameter variable in the given system.
+Check if the given sym is a parameter variable in the given system. Defaults
+to `false` if not implemented for the given system/container type.
 """
 function is_param_sym end

src/symbolcache.jl

@@ -7,17 +7,20 @@ end
 
 independent_variables(sc::SymbolCache) = sc.indepsym
 independent_variables(::SymbolCache{S,Nothing}) where {S} = []
+is_indep_sum(::Any, _) = false
 is_indep_sym(sc::SymbolCache, sym) = any(isequal(sym), sc.indepsym)
 is_indep_sym(::SymbolCache{S,Nothing}, _) where {S} = false
 states(sc::SymbolCache) = sc.syms
 states(::SymbolCache{Nothing}) = []
 state_sym_to_index(sc::SymbolCache, sym) = findfirst(isequal(sym), sc.syms)
 state_sym_to_index(::SymbolCache{Nothing}, _) = nothing
+is_state_sym(::Any, _) = false
 is_state_sym(sc::SymbolCache, sym) = !isnothing(state_sym_to_index(sc, sym))
 parameters(sc::SymbolCache) = sc.paramsyms
 parameters(::SymbolCache{S,T,Nothing}) where {S,T} = []
 param_sym_to_index(sc::SymbolCache, sym) = findfirst(isequal(sym), sc.paramsyms)
 param_sym_to_index(::SymbolCache{S,T,Nothing}, _) where {S,T} = nothing
+is_param_sym(::Any, _) = false
 is_param_sym(sc::SymbolCache, sym) = !isnothing(param_sym_to_index(sc, sym))
 
 Base.copy(VA::SymbolCache) = typeof(VA)(