Final Etot extractor is necessary in DFTIO

To serve wider applicaitons and multiple loss functions formula in Machine Leanring Electronic Simulaitons, it's necessary to implement an Extractor of Final total energy `Etot` in DFTIO.

This extractor would at least serve the following DFT packages:
- ABACUS
- SIESTA

This extractor should support extraction of Etot in various scenarios like static calculation and MD.

The extracted `Etot` would be saved as a scalar for single structure and a list for a trajectory.