FRQ: generate continuum trajectory from particle trajectory

[kay-ro]

Proposed new feature or change:

The function continuum.gkde allows to convert particle data to continuum-data.
It would be very useful to make analysis starting from this much more accessible:
If the user can convert a particle trajectory with a command to a continuum/Field trajectory, the analysis functions that work for fields but not for particles would be open.

• For a first step it would be sufficient to only convert the density/one property.
• If very easy to implement, think about making it a general method for all available particle data.