Implement commonly used rocket propagation stop conditions

[alfonsogonzalez]

The Rocket class models a rocket trajectory by a series of phases. A phase end can be triggered by many events, all of which will be implemented in this issue. So far here is the list:

• Maximum altitude (trigger to begin pitchover phase)
• Minimum mass (trigger to stop thrust)
• Maximum dynamic pressure (trigger to throttle down to reduce max q)
• Rocket has reached orbit (won't crash to Earth surface assuming two-body dynamics)
• More conditions likely to be added