Support to do gradient calculation on a loss function from trajectory

[ice-hanbin]

Add DiffTraj to provide a support to do gradient calculation on a loss function from trajectory,
write the doc, example and test about DiffTraj above.

[KuangYu]

Please futher improve the document:

1. Be more detailed for the new classes an new functions, and the design logic behind them
2. Include links to the example file
3. Fix the index of document