Add an optional speed modifier to an entity

Currently all mobile entities travel at two fixed rates of speed depending on if they are walking or running.

There are many cases when it could be preferred and more realistic to be able to apply a modifier to this base value to allow for variable velocities. This could be dependent on the type of mobile, a horse will easily out-run a human, or it could be applied in a more specific manner, some humans run faster then others based on physical attributes. Or a player or creature might be under a magical effect.