Spawning entities at runtime bug

[UltronDestroyer]

Answer the following questions:

• what are you trying to do?
  Spawn an agent at runtime

• what is the problem and how can it be recreated?
  When is 0 in the XML file, it sets max_num_entities in SimControl.cpp to be 0. Therefore, it is impossible to spawn agents without causing a segfault.

• what scrimmage commit are you on? You can see this with git rev-parse HEAD
  Head

• describe any changes you made to scrimmage.
  I changed max_num_entities to 100 for example, and it continued on without segfaulting

Changes proposed:
Allow there to be a parameter in the XML file such as "max_num_entities" which is separate from count and outside of the entity tag such that spawning entities at runtime is possible

The max_num_entries variable resides in SimControl.cpp and gets set to 0 if the total count of 0 is 0 for all entities which means the RTree initializes with 0, eventually providing a segfault

[SyllogismRXS]

I made some changes to how the scrimmage RTree is constructed in SimControl. I merged the changes into master after they passed Travis CI tests. See the changes in commit bdebe85

Let me know if this works for you.

[UltronDestroyer]

Thank you!

I will get back to you tomorrow on the changes.