Namespace cleanup

BREAKING CHANGE:

• moved library-specific constant #define into library class constants
  • library-specific constants declared as global #define polluted global namespace and interfered with other libraries
• if this change breaks your code, rename PID constants in your code as follows:
  • DIRECT to PID::DIRECT
  • REVERSE to PID::REVERSE
  • P_ON_M to PID::P_ON_M
  • P_ON_E to PID::P_ON_E