Adds an entity editor to the Testbed for live updating and observation of PlayRho entities

[louis-langholtz]

Description - What's this PR do?

• Adds an entity editor to the Testbed for live updating and observation of PlayRho entities.
• Fixes assertion code in the math Cross function.
• Adds defensive code to CalcGravitationalAcceleration to avoid division by zero.
• Adds a couple more unit entries to facilitate usage of units.
• Adds mutable Joint visitation.
• Adds ==, != support to Filter class.
• Refactors joint methods that took Frequency input into methods that returned member data and free functions that used them along with the Frequency as free function substitutes for the former.
• Moves entity based UI code out from Test and into main.cpp.
• Adds lots more entity UI code into main.cpp to provide the new Entities Editor window to the Testbed.

Screenshot

Here's a screenshot of the editor in action for the Joints Overview test...

[coveralls]

Coverage decreased (-0.2%) to 93.682% when pulling 0153dcd on enhancements-for-testbed into c21d454 on master.

[coveralls]

Coverage increased (+1.3%) to 95.253% when pulling a00501e on enhancements-for-testbed into c21d454 on master.