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.
Yes but not long ago this was an invert kwarg. As long as we're reinventing
the API, let's do it consistently for all classes.

On Tuesday, June 24, 2014, Thomas A Caswell <notifications@github.com>
wrote:

> In pims/ffmpeg_reader.py:
>
> > @@ -91,11 +91,12 @@ def available():
> >
> >  class FFmpegVideoReader(FramesSequence):
> >
> > -    def __init__(self, filename, pix_fmt="rgb24", process_func=None):
> > +    def __init__(self, filename, process_func=None, pix_fmt="rgb24",
>
> re-ordering kwargs is bad.
>
> —
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> <https://github.com/soft-matter/pims/pull/52/files#r14129621>.
>