Grand canonical ensemble and unit testing.

[rpcollanton]

• Modified >>, << operators in species to match their declaration.
• Added safety checks in MixtureTmpl accessor functions to give clearer exceptions when a user enters an erroneous param file.
• Substantially modified AmIterator: added thorough commenting, renamed variables for clarity and to parallel Arora 2017 paper, using safer System::setWBasis function, and enabling grand canonical functionality.
• Added thorough testing for pure grand canonical ensemble solutions and blends, including testing chemical potential shifts.

[dmorse]

Ryan - In the tests, please change

if (error) { UTIL_THROW(...) }

to

TEST_ASSERT(!error)

Rationale: Failing a TEST_ASSERT throws an exception that is caught by the unit test framework, stopping operation of that test and causing it to be recorded as failed, but doesn't cause the program to exit or crash. Throwing an uncaught exception crashes the unit test program. Marking the test as failed and continuing seems more appropriate. I'll accept the pull request after you make another small commit to the same branch with this change.

I don't think you need to do anything other than commit the change and tell me you've done so. I believe the pull request points to the current state of the originating branch, and so is automatically updated if that branch updates. Let's try this out in any case to make sure we both understand how to make and respond to requested changes in a pull request.

[rpcollanton]

Dave -- in SystemTest.h, I used TEST_THROW instead of UTIL_THROW. This halts execution and records a failure in the same way as TEST_ASSERT but allows a custom message. I felt it was appropriate since a message "!error" is somewhat nondescript. With this in mine, if you'd like me to change it still, I can do that!

[rpcollanton]

Also, I have messed around with PRs a bit and can confirm that it points to the current branch state so that if I make a new commit, it will be added to the PR.