Warn the user when simulated patterns won't match geometry file

The libcrystfel routine for writing an HDF5 file (image_hdf5_write) is quite severely limited. It doesn't understand path placeholders or data blocks with more than 2 dimensions. Fixing this would be ideal, but would be quite difficult. I don't think the effort is justified right now. Instead, pattern_sim should warn the user if the DataTemplate contains things that image_hdf5_write can't handle. The warning should be that the files so written won't be readable using the same geometry file.