[python] Add worldEntity util

[chapulina]

When I suggested removing the configure callback until we found a way to handle the EventManager, I didn't realize this would make the example kinda meaningless (i.e. not checking world gravity). Here's a suggestion to wrap the worldEntity function so we can get the entity in an update call, without relying on Configure.

Also added a few more suggestions:

• Move example to its own directory in examples/scripts. This way we can add other python examples there later too.
• Added a README to the example
• Minor updates to remove configure and left-over comments.

[codecov]

Codecov Report

Merging #1308 (1e03834) into ahcorde/python/gazebo (5619121) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@                    Coverage Diff                    @@
##           ahcorde/python/gazebo    #1308      +/-   ##
=========================================================
- Coverage                  62.25%   62.25%   -0.01%     
=========================================================
  Files                        278      278              
  Lines                      23203    23203              
=========================================================
- Hits                       14446    14445       -1     
- Misses                      8757     8758       +1     

Impacted Files	Coverage Δ
include/ignition/gazebo/EventManager.hh	79.16% <ø> (ø)
src/EntityComponentManager.cc	88.26% <0.00%> (-0.13%)	⬇️

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