Unity Plugin: compiler directive to enable builds and synchronising C# scripts with MuJoCo

[Balint-H]

Description

Two small additions:

1. The plugin's Editor scripts were not ignored during build, leading to unresolved references preventing standalone builds. The scripts that prevented the build were wrapped with the following directive:

#if UNITY_EDITOR
//Component Editor Script
#endif

Behaviour is not impacted in the editor, but builds are now successful. Fixes #245.

2. An event was added to MjScene that allows C# scripts, such as custom controllers and agents to subscribe handlers that are invoked between mj_step1 and mj_step2. This can improve the stability of certain actuator controllers, and can guarantee order of execution instead of relying on separate FixedUpdate()s. If no such handler is subscribed, then mj_step is executed as usual. Pointers to the simulation data for event handlers are included in a custom EventArgs. Related to #233.

[jespersmith]

I propose an alternative fix to the compiler directives, by configuring the .asmdef in the Editor folder correctly

https://github.com/deepmind/mujoco/pull/303

This avoids having to create a lot of #ifdef directives and keeps the code cleaner overall.

[Balint-H]

I edited this pull request to only add the events to MjScene that can be used to synchronise behaviour with C# scripts, and removed the edits to the Editor scripts, as I believe #303 handled that part better. Furthermore I added events that happen before and after mjStep, as these can be used to control order of execution independent of project settings, useful for example in behaviours in MlAgents environments.

[Balint-H]

I will submit a separate pull request for the event synchronising functionality from a branch thats more in sync with the deepmind main.