Why is IQS not building after April 2021 release? Add "iqs" namespace.

Hi all, I opened this issue as a way to communicate with all users.
In April 2021 we released IQS 2.1.0
This version includes new features and examples (see CHANGELOG for a short list), but also some code cleaning.
As part of adopting good coding practices, we collect all IQS class and methods in namespace "iqs".
If you have problems re-building previous projects, this may be the reason.

The fix is simple. In C++, when declaring a new QubitRegister object just use:
QubitRegister(...) ---> iqs::QubitRegister(...)