Velocity unsteady

[MaikFZJ]

compute_individual_velocity calculates unsteady velocity depending on frame_step at the beginning and end of the trajectory. The interval does not increasing gradually and has the current frame in the centre.

[schroedtert]

After discussion:

• leave current version in
• add new velocity methods, switch with parameter
  • exclude parts where not the whole frame interval is valid
  • use an adaptive approach (increase frame interval till it reaches full width)