Add multiple single parts to make it a multi-part method

[zhyt1985]

This algorithm takes a vector layer with single-part geometries and generates a new one in which all geometries are multipart. Input features which are already multipart features will remain unchanged.
This algorithm can be used to force geometries to multipart types in order to be compatible with data providers with strict single-part/multi-part compatibility checks.
I really need this feature, thanks a million.