Reflection_Engine: Add a method to query subtypes of a type

[pawelbaran]

Description:

In some cases (mainly in adapter actions - see BHoM/Mongo_Toolkit#166) it is needed to get all subtypes of a type, so would be good to add a method that does it.

[FraserGreenroyd]

I think @alelom did something similar which he's put on his latest Adapter PR @pawelbaran

[FraserGreenroyd]

Ah no, forgive me, he did the opposite - getting all base types of a type, not the sub types... Though he still needs to PR his change up to Reflection Engine from my review 😄