Custom rdds:multitif fixes

Two small but probably important fixes in the wake of PR #36:
* fix for tif file handling in case where there's only a single strip or tile in the image. (As it happens, tifs generated by PIL itself fall in this category.)
* fix series block size calculation from tifs to include number of time points (image files) in the estimated block size. Previously number of images was just omitted, leading to blocks substantially bigger than they should have been.