Characterization with tuple valued diameter

- Characterization is turned off when pixels are not square or cubic
- Characterization works again with tuple valued diameter
- Bug in peak fuser (https://github.com/soft-matter/trackpy/pull/143) +
big error by myself in same code
- Bug in roi: threshold should default to None, else float pictures will
be made completely black.
- Slices named 'square' are now named 'rect'
- Validate_tuple function to utils + check in masks and bandpass