Fix/tests

[nathanhhughes]

@Schmluk Found a somewhat workable solution for handling the full-stream precision that yaml-cpp uses under the hood for floating-point representations. Other notes:

• Brings gtest_discover_tests back (I'm hoping I found the right arguments to avoid the weird linking issue that happens in catkin that made me remove it originally) so we can use ctest
• Fixes parsing negative numbers on new-ish verisons of yaml-cpp (you can't do as<unsigned> on a literal with a negative sign)
• Adds some minor fixes for unit tests that were previously passing because of global config settings set in other tests (which we should maybe more explicitly fix, but gtest_discover_tests isolates each test into a single executable already).

Lmk what you think about the float formatting. We can also talk in-person about the what else I tried, I don't think there's an easy way to get around the edge-cases to my solution (i.e., string fields that are look like really long floats) without a much more substantial overhaul to support tracking which scalars are floats.

[Schmluk]

Looks great! Probably a good thing to have floats formatted to human readable sizes by default 👍