Add more test coverage

[ewu63]

Description

Unit tests

• test that the Solution object contains the right info
  • objectives
  • constraints
  • variables
  • can be correctly cold-started (document this!)
• FD/CS tests

Integration tests

• MPI tests
  • fake COMM object works on single proc
  • optimization in parallel runs fine, with only the root proc running the optimization itself but calling the objective/sensitivity functions on all procs
  • parallel DV/constraint adding works as expected
• other optimization problems?
• FD/CS tests