v4: Add possibility of set custom assembly name

[bielu]

Hi Stephan, As I was playing little with v4 models builder, I noticed it would be helpfull in few cases to setup custom asssembly name, as namespace not always fit the assembly name. I tried to make changes in manner of supporting both approaches and by default falling back to namespace.

[zpqrtbnk]

For my own memory: prior to this PR the models assembly name was aligned with the models namespace. For some reasons, some ppl may want a different assembly name. This PR does not change any default behavior but add the possibility to customize the assembly name.

Sounds good.