Remove instances of `std::endl`

[Woolfrey]

Apparently it is faster to use:

std::cout << "\n";

to terminate rather than:

std::cout << std::endl;

Need to check:

• [x] Control classes
• [x] Trajectory classes
• [x] Model classes

[Woolfrey]

Almost zero instances of std::endl found.

Retained std::cerr << std::endl;. Need to double-check if this is necessary.