Environment variants with continuous actions and multi-objective, bug fixes.

• fix a bug in generating discrete actions from continuous actions
• fix more bugs related to changes in gym's latest versions
• new intersection-env variant with continuous actions
• add longitudinal/lateral/angular offsets to the lane as part of the kinematics observation's features
• add more configurable options for reward function and termination conditions
• add configurable min/max speed for continuous actions
• bug fix for reward computation in the multi-agent setting
• add get_available_actions for MultiAgentAction
• fix various deprecation warnings
• add a multi-objective version of HighwayEnv

Huge thanks to contributors @zerongxi, @TibiGG, @KexianShen, @lorandcheng