specify target columns for event properties

[dkrako]

Description of the problem

Currently columns that are created by compute_event_properties() will always match the names of the respective property.

For example, if someone wants to compare mean and median dispersion, this is not possible.

Description of a solution

Instead it should be possible to specify the target columns for the calculated event property.

Context

Related issues:

• raise helpful error message if event property column already exists #1041
• support recomputing event properties #1042