Several updates:

- Tweaks to optimize.  added href and function as ResponseFunction tags
- pprepro ignores evaluation errors if in {} tags - presumeably because
  they will be supplied by Permutation or Optimization handlers.
- inpparse modified as per above
- respfcn returns None instead of np.nan for failed evaluation
- major update to docs
- optimize and permutate both got some general update
- main/physics/inpparse modified to allow for multiple Physics elements
  in an input file