Add next observations to the Trajectory Buffer

[thomashopkins32]

For ICM, we will need to have the trajectory buffer store the next observation in addition to the current one. This complicates things a bit as we will now have 4 different versions of an observation:

• The current full observation
• The region-of-interest of the observation
• The next full observation
• The region-of-interest of the next observation

This makes me revisit the idea that we need to have the region of interest computation outside of the agent. Maybe we can keep it inside somehow?

[thomashopkins32]

This will be handled in #14