Maml

* maml is now compatible with both mlip-2 and mlip-dev

* Fix SNAP stress components
* Fix unexpected test outcome in BOWSR and symbolic regression

* bug fix
* compatibility with new n2p2 and mlip software
* upgrade rfxas

* bug fix
* add atomsets models

* major refactor
* move mlearn potential energy surfaces to apps
* fix potential problem of rotated structure in lammps
* add rfxas app
* add initial symbolic regression app
* add megnet-based describers

* initial release