Wish: Dependent elements should inherit IsExternal attribute value

There are a number of dependent elements that would benefit from inheriting their ```IsExternal``` attribute value from their respective parent / host:
- Stringers of assembled stairs
- Stair-hosted railings
- ~~Curtain wall panels~~
- ~~Curtain wall mullions~~

All of these report their ```IsExternal``` values as ```False```, regardless of the ```Function``` parameter value of their respective host / parent element.

Tested in 22.4

## Expected behaviour
The ```IsExternal``` property value should be inherited from the ```Function``` parameter of the host / parent element if no override (see below) is present.

## Workaround
(Yes, this works as expected, but it's a nuisance and users keep forgetting...)
- Create distinct interior / exterior railing types
- Override using ```IsExternal[Type]``` shared parameter

Start by tracing how IsExternal is computed for assembled-stair stringers and stair-hosted railings, using the host or parent Function parameter and the IsExternal[Type] override described in the issue. Done means those dependent elements inherit the host value when no override is present, while the documented override behavior remains available.

Written by the indexing model from the issue text.