Make "availability" an input value where applicable

I have noticed that many features, such as CzmlEllipsoid, Polygon, Polyline, etc. have a variable called availability that determines the time frame for which that feature will be visible in the simulation. I haven't been through all the code yet, but it looks like that for many/all of the features with this member variable, the value is set by the time interval contained in the header information, which by default is the entire time frame of the simulation. I would like to change this feature to one that can optionally be user-chosen, and is only set by the default header interval value if the user does not set it as something else. This would allow users to set these features to appear in the simulation only when their presence is actually desired.