New print assertions

[varunagrawal]

This PR adds new assertion functions to be able to test print outputs in a clean, functional way, reducing copy-pasta.

With these assertions, we can test for consistent printing across platforms, to improve the overall GTSAM experience.

• assert_stdout_equal checks for the output when using std::cout <<.
• assert_print_equal checks the output when using a class' print function, e.g. pose.print().

[varunagrawal]

I came up with this PR because I found myself reinventing the stringstream-cout-redirect wheel too many times. 🙂

[jlblancoc]

@varunagrawal What about adding all print() methods an extra optional parameter of type std::ostream, defaulting to std::cout, and which could be set to a local std::sstringstream in these functions? That idea has been around in my head for a long time, but it would imply touching many (all!) print() methods...
Thoughts?

[varunagrawal]

I have been thinking about the same thing, but I believe the better approach would be to overload the << operator for all classes that support the print function. Then the use of a stringstream would be trivial.

I did contemplate capturing the output in a stringstream, but there were 2 problems I wasn't sure about:

1. For different systems and compilers, it made sense to perform the redirection so that we were reproducing real printing conditions, rather than a pseudo-print. The last thing I wanted was the headache of users complaining about printing being bad.
2. The print() function is what we use in the wrapper to print details of an object, so I wasn't sure how adding an optional parameter would affect the wrapper's operation. @ProfFan is the wrapper expert and I just have working knowledge of it. Given that Fan is pretty busy these days with SwiftFusion, I opted out of that extra pain.