Preparation for multi-component systems

[jvita]

• Add the option for condensing SVs for single-component systems
• Figure out how to modify tree.eval() to handle proper application of U' terms to force vectors. This will involve modifying evaluator.evaluate() to not sum over the last dimension, then letting the tree apply the proper branch depending on the atom type. Note that since sv.loop() should return values with the same format as evaluator.evaluate(), this means that you will also need to modify sv.loop() to have an extra dimension in the force matrix.

[jvita]

The following decisions were made to address this issue:

• There will be a separate tree for each host element type
• There will be no parameter sharing between any nodes (not even FFG_AA and FFG_AB in the same chemistry tree).