Description of the error
The CP2K converter execution is blocked unless the user inputs a velocity file, even though it should be enough to input the positions and unit cell files.

Describe the expected result
The velocity file should be optional.

Describe the actual result
Unless a valid file name has been specified, the GUI makes it impossible to start the converter.

Suggested fix
Optional input file widgets already exist in other converters, such as the MDTraj converter. A similar approach should be implemented.

Additional details
In principle, we should also have a slot for an optional input file containing forces per atom, since our trajectories can store these too.