Atmosphere is not pulled from Shower HDF5

Currently the information about the atmosphere is not read from the HDF5 file when creating a SlicedShower object. This forces the user to know the atmosphere used for the simulation and provide it as a init argument to the TemplateSynthesis constructor. Probably it would be better to redesign the code such that the make_template() function creates the atmosphere object, using the information from the origin shower that is provided.