Fix oversights

Missing test coverage for `frame_no` and `process_func` let in some API inconsistencies and some functionality missing from some classes.

This adds tests and makes several important (but I think uncontroversial) fixes.
Is there a tricky way to enforce consistent init args among classes, akin
to what you did with abc?

On Tuesday, June 24, 2014, Thomas A Caswell <notifications@github.com>
wrote:

> In pims/tiff_stack.py:
>
> > @@ -59,7 +59,7 @@ def class_exts(cls):
> >          return {'tif', 'tiff',
> >                  'stk'} | super(TiffStack_tifffile, cls).class_exts()
> >
> > -    def __init__(self, filename, dtype=None):
> > +    def __init__(self, filename, process_func=None, dtype=None):
> >          self._filename = filename
>
> Fair enough.
>
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> <https://github.com/soft-matter/pims/pull/52/files#r14146737>.
>