Fix lsm

The Problem: LSM files store image data in even nodes only.

This adds a special LSM reader that subclasses `TiffStack_libtiff` and overrides `get_frame` so that Frame `j` is sought in "directory" `2 * j`. Testing `v[0]`, `v[1]`, and `v[len(v) - 1]` all work as expected. Automated unit tests will have to wait until we solve the data issue.

Also, since this reader is not a direct subclass of `FramesSequence`, I amended the `open` command to search recursively for *all* subclasses and (sub-sub-...-sub-classes) of `FramesSequence`. Hence,

    open('filename.lsm')

works.