Unit tests for Trajectory API

[cnboonhan]

This PR is to introduce unit tests for the Trajectory API in rmf_core. I have flagged possible failing tests using the keyword FLAG and commented them out.

Where possible I have tried to check move and copy constructors and their side effects. I may not have done them 100% correctly, would be good to double check how valid the tests are.

The tests are written BDD style: the logical meta-structure is as follows:

• For each class, starting from the most atomic,

• Test creation of the class,

• Test the class functions

[mxgrey]

Here's a preliminary review of what I've looked through so far. I'm only about halfway through test_trajectory.cpp at the moment.

These tests are really fantastic and comprehensive, so thanks for putting all this time and thought into them. You'll find some specific remarks about specific lines below, but I'll also add one general remark:

I strong recommend making liberal use of the const qualifier in your variable declarations. You should always default to declaring variables as const unless you definitely need to modify their values later. That's not anything specific to writing unit tests, but more of a general C++ programming recommendation.

[mxgrey]

Most of feedback is really minor, just small tweaks to improve the code quality.

Thanks again for writing all these tests!

[cnboonhan]

will add const declarations, and follow the pointer conventions, in a final pass before merging.

[cnboonhan]

I have added many consts ( the best consts ). Sorry if I have missed out anything: i am assuming that as everything is marked as resolved, that I do not need to track them.

If everything is ok, I would like to merge this as well as delete the refactor traffic_controller_add_tests branchwhich is now redundant.

[mxgrey]

Those are some very strong consts, some would even say the strongest.

Great job with these tests; I'm happy to merge it now.