Changes following tcaswells notes

- removed memoizer
- removed 'modes': just use Bioformats2D if you want 2D and Bioformats3D
if you want 3D
- get_frame is now a wrapper for _get_frame, which is different in 2D
and 3D. both versions use _get_frame_2D
- changed name 'multipoint' to 'series' to be consistent with bioformats