check embedding selection
Details
we want to check if the embedding that the user requests is present in the assay. If not return error. If more than one with that name return last entry of that name and warning. Default is last that will just take the last embedding created which should be stored in the active slot in the vesalius_Assay object. We also want to be able to select which dimensions we want to return. We make sure that those dimensions can be extracted from the embedding data frame.