Resolution functions plots do not account for mosaic spread

[tgperring]

The covariance matrices returned by tobyfit_DGdisk_resfun_covariance and tobyfit_DGfermi_resfun_covariance do not account for the sample mosaic spread. Consequently the resolution function plots returned by resolution_plot do not account for this component of the resolution function. The covariance matrices need to have the mosaic spread contribution added for correctness and for compatibility with tobyfit. The functions tobyfit_DGdisk_resconv and tobyfit_DGfermi_resconv do account for mosaic spread.