Document use of Activator.CreateInstance for analytical interfaces

[IsakNaslundBh]

As mentioned:

https://github.com/BHoM/BHoM_Engine/pull/1933#pullrequestreview-469303030

https://github.com/BHoM/BHoM_Engine/pull/1933#pullrequestreview-469473041

Need to document when Activator.CreateInstance need to be used.

To also keep in min is potential for sppedups:

https://codingsolution.wordpress.com/2013/07/12/activator-createinstance-is-slow/

@FraserGreenroyd @al-fisher