bfconvert: fix conversion of individual series

This ensures that the image width and height are set correctly if only
one series is being converted.  Previously, attempting to convert a
series with dimensions different from series 0 would have resulted in a
FormatException.

To test, verify that ```bfconvert -series 1 input-file output-file.tiff``` works regardless of whether or not series 0 and series 1 have the same image dimensions.  Most HCS datasets should have the same dimensions; most pathology (e.g. SVS) datasets should have different dimensions.