restructure the files for reaxff, add frozendict dependency, add ase …

…dependency for testing

.github/workflows/build.yml

@@ -41,6 +41,7 @@ jobs:
         pip install pytest-xdist
         pip install pytest-cov
         pip install netCDF4
+        pip install ase
     - name: Set Swap Space
       uses: pierotofy/set-swap-space@master
       with:

jax_md/__init__.py

@@ -25,10 +25,6 @@
 from jax_md import util
 from jax_md import io
 from jax_md import rigid_body
-from ._reaxff import reaxff_energy
-from ._reaxff import reaxff_interactions
-from ._reaxff import reaxff_forcefield
-from ._reaxff import reaxff_helper
 try:
   # Attempt to load colab_tools if IPython is installed.
   from jax_md import colab_tools