Added takeoff and land functions to the spiri api

Using the takeoff and land functions depends on having a spiri_motion_primitives_server (found in spiri_navigation repo) running. Without one, it will hang forever waiting for the server to come up.


We can easily add parameters like height and speed to these functions but I didn't do it yet because I'd like to talk about the structure of the API before I go adding a bunch of stuff to it. If many motion primitives are added, it will be smarter to not create a new client each time one of the functions that tries to use one is called.