Special time variable support

[schillic]

This is a follow-up issue of #256. See https://github.com/JuliaReach/Reachability.jl/pull/228#issuecomment-426927132 and https://github.com/JuliaReach/Reachability.jl/pull/228#issuecomment-426954466 for the discussion.

To have more exact time plots for hybrid systems, we should offer an option to include a time variable (e.g., put it in a new block, etc.) and ignore the time tracking in this case for the projection.

[mforets]

Another application is to define a property that involves time in a subset of the whole time horizon. An example is:

After time t [seconds], the height x [meters] should stay above 0.9m.