Fix existing tests for changes made as part of the igtools update

[jranalli]

This updates the tests for the ideal gas classes to match the basics of the new functionality.

• Remove the invargs error check (there's not an error with too many invargs anymore)
• Update the test entropy values for air to match the new entropy calculation including smix
• Change the h&s and e&s tests to test values rather than testing for them to error

It's important that you merge these tests in, because there are several major functionality-breaking bugs that were introduced in the edits to the ig classes. I have identified many of them and can start to propose fixes, but we've got to update the tests first.

TODO:

• ~~Write tests for the new igtools module~~ comparable to current state of test_ig at least
• ~~Incorporate testing of Gibbs & Helmholz and speed of sound for all substances~~ done
• ~~Look for other tests that have been invalidated~~ double checked, looks ok