PR to master: openKIM feature

Now, we merge the openKIM functionality in the periodic relaxation branch into master. Commits: 005823eb a37efe82 59844366 24fdccb2 ca9df2d9 19caae81 40e14cc7 e4eb7b3e f38dc104 030ee35b 1e8d1ca5 ae2cecbf 057660d1 55693fbf c02b7bf9 696420eb 105f5dfb 895c0e30 b67210e3 d0c855b0 26bd0861 e4022f5b 4861ae59 2d45d8d0 d994c460

Open tasks:

• Change material back to Finnis-Sinclair copper
• #65 (closed) Modularize preprocessor for test coverage (@brgerhar)
• Is a smaller mesh/fewer iterations enough to test the behavior? (This would enable unit tests.)
• How can such meshes be generated (@ssaxena)?
• Unit test (simplifies testing details):
  • Computing energy works via KIM
• Integration test (simplifies debugging):
  • Idea: In-memory mesh, few iterations of relaxation