mini-batch training

[brucefan1983]

This PR aims to improve the training results when using mini-batches, based on the following tricks:

• Increase the default regularization by a factor of $\sqrt{10}$.
• Set up an upper limit of the learning rate: $\sigma_{\rm max}=0.01$, which is also the initial learning rate $\sigma_0$.
• Prepare the mini-batches according to energy/atom values, maximizing the diversity for each batch.