fix to preserve input image shape upon executing `random_walker`
PEP8 fixes; remove unneeded pyamg import; add `data` shape check
Use `force_copy` argument of `img_as_float`

Also three PEP8 fixes
fix: remove unnecessary forced copy
add assertion checks to ensure `data` shape is unchanged
Merge pull request #500 from JDWarner/fix_preserve_rw_input_shape

BUG: Preserve input image shape upon executing `random_walker`.