Ani 2x (#480)

* Adding ANI-2x parameters

* Moving 2x to seperate branch

* Adding ANI-2x to new branch

* Adding 2x network

* Update models.py

* Update api.rst

* Update __init__.py
Co-authored-by: Christian Devereux <cdever01@ufl.edu>
Co-authored-by: Farhad Ramezanghorbani <farhadrgh@users.noreply.github.com>

torchani/resources/ani-2x_8x/train1/networks/ANN-C.nnf-l4.bparam

+;;
\ No newline at end of file

torchani/resources/ani-2x_8x/train1/networks/ANN-Cl.nnf-l4.bparam

+T9
\ No newline at end of file