FRQ: update/append trajectory

[kay-ro]

Proposed new feature or change:

To extend an existing trajectory with additional simulation data, it would be useful to be able to append frames to an existing trajectory. Since the trajectory object already has similar properties as a list, we can define an append methods similar to the one for lists. This also allows to update a trajectory for extended data.

ToDo:

• implement the append method in BaseTrajectory
• add extend=False keyword to LammpsReader and ContinuumReader
• if True, the reader checks whether new dump files are in the directory of the h5amep file and if so, it adds those to the h5amep file
• make compatible with minor release!
• decide on approach
  • implement a update method in BaseTrajectory or
  • alternative: add update keyword to load.traj(...)
• write a short test method for the unit tests
• update documentation
• write short example(s)