Issue - Allow user to create own CzmlModel

There are several classes (Spacecraft, ManeuverSequence, perhaps others) that require the generation of CzmlModel objects, but do not allow the user to input the CzmlModel into the class directly. This can cause problems because it does not allow the user to specify values such as sizing for the model, which in turn sometimes means the models in the CZML output files are too large/small, or have other issues. I would suggest that the code either allow the user to choose between putting in a CzmlModel or a path to a model file, or simply move to requiring the user to put in a CzmlModel directly.