fixup! refactor: add new *_symbols methods, corresponding singletons,…

… update tests

docs/src/usage.md

@@ -46,7 +46,7 @@ sol[x]
 ```
 
 This also works for arrays or tuples of variables, including observed quantities and
-independent variables:
+independent variables, and for interpolating solutions:
 ```@example Usage
 sol[[x, y]]
 ```
@@ -55,6 +55,18 @@ sol[[x, y]]
 sol[(t, w)]
 ```
 
+```@example Usage
+sol(1.3, idxs=x)
+```
+
+```@example Usage
+sol(1.3, idxs=[x, w]) # cannot use tuples of variables for interpolation
+```
+
+```@example Usage
+sol(1.3, idxs=[:y, :z])
+```
+
 If necessary, `Symbol`s can be used to refer to variables. This is only valid for
 symbolic variables for which [`hasname`](@ref) returns `true`. The `Symbol` used must
 match the one returned by [`getname`](@ref) for the variable.