[ASE interface] Logger's compatibility with ASE Trajectory

[YuuuXie]

Currently the data collection is a bit duplicated.

1. We have a unified *.log file which contains everything about the MD traj and training info, this file is of the same format with primitive GP-OTF log file (without ASE)
2. We also have an otf_data folder, containing data files storing GP positions & forces, DFT frames, etc. separately. And this part is duplicated with (1)
3. If ASE's own trajectory is on, both the GP and DFT files will be written to it. Consider if we need to separate the two.

One possible solution:

1. For MD data (without DFT), we set an option that either the trajectory can be stored the same as otf_data folder, or use ASE trajectory. The only issue for ASE traj is recording the uncertainties, we might need another logger.
2. Keep DFT data the same as otf_data (.xyz files), or write as ASE Trajectory (set an option for the two)

[YuuuXie]

A few discussions and temporary solution, refer to #133, #136

[YuuuXie]

close via #214