Adding documentation about multiple physics engines

[jharwell]

Hello, I'm currently trying to put some docs for using multiple physics engines into the query output for the 2D/3D engines, so ARGoS can be a bit more self-documenting. Looking at the multiple physics engine example in the ARGoS examples vs. the code in physics_engine.cpp, I have the following questions:

• Is it possible to have physics engines which are not assigned a closed polygon in the 2D plane ? The code allows this (it only checks for polygon closed-ness if there are more than 3 vertices specified in the <boundaries> section), but I'm not sure when this would be useful/needed. Has this been used before?

• Is it possible to have physics engines without a defined "top" or "bottom"? Are these attributes that can be omitted for 2D physics engines but need to be present for 3D engines? The code does not make a distinction, and I would think that you need to define both in order to specify multiple physics engines in 3D at least, if not 2D.

• Is it possible to set the elevation attribute of a 2D physics engine? The query results say yes, but parsing that attribute does not appear anywhere in physics_engine.cpp.

• Is it possible to use multiple 3D physics engines in addition to multiple 2D physics engines? Can 2D/3D engines be mixed and matched within the same simulation, so long as the engine boundaries do not overlap? The lack of distinction between 2D and 3D in physics_engine.cpp XML parsing and my knowledge of ARGoS implies "yes" to both of the above questions, but I wanted to be sure before I add something to the docs to that effect.

@ilpincy, @allsey87 Thoughts?

[jharwell]

@ilpincy,@allsey87 gentlest of bumps on this. Do you have any answers to my questions above? I'm enough enough of an argos expert to be able to figure them out from just looking at the code. Thanks!