bdtick function does not recognise SIX, Borsa Italiana and Xetra Exchanges

[RamonWill]

Hi, I noticed that for the blp.bdtick function certain exchange codes aren't registering i.e. "GY", "IM", "SE".

GY = Xetra IM = Borsa Italiana SE = SIX

I realized that this is due to these exchanges missing in the assets.yml and exch.yml.

I have a fix that i can put in place when i get the time.