Replace Unit with UnitInfo object

[LLipter]

This pull request replaces unit in the environment with UnitInfo object. It also updates all related python bindings.

[rdiankov]

@LLipter there was a point in UpdateFromInfo that was assigning _unit in production, which I just changed to check and throw an exception. can you do the same in this branch and test? Thanks

[LLipter]

@rdiankov Sure, I also update my MR on webstack to throw an error in this situation

[rdiankov]

@LLipter did you actually test, this simple scene fails:

{
  "bodies":[{
    "name":"C",
    "links":[{
      "name":"L",
      "geometries":[{
        "name":"G",
        "type":"cylinder",
        "radius":1,
        "height":1
      }]
    }]
  }],
  "unitInfo": {
    "lengthUnit":"mm"
  }
}

[rdiankov]

thanks~