BLD New release: 1.2.2

Sobel errors were causing test failures on newer numpy releases. For
many functions, passing float16 or float128 arrays would cause some
incomprehensible errors.

Full ChangeLog
    * Add minlength argument to labeled_sum
    * Generalize regmax/regmin to work with floating point images
    * Allow floating point inputs to ``cwatershed()``
    * Correctly check for float16 & float128 inputs
    * Make sobel into a pure function (i.e., do not normalize its input)
    * Fix sobel filtering