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Use 1-D structured array fields for position-based kernels in `ndmeasure` (#79)

* Use N-D structured array field in `center_of_mass`

Instead of using multiple structured array fields to pass back the
different coordinate components of the center of mass, use one 1-D field
with the size being the number of components. This simplifies the array
handling a bit and avoids the need to split apart and restack the array.

* Use N-D structured array in `maximum_position`

Rewrite `maximum_position` to use a structured `dtype` with a 1-D field.
This allows the unraveled position to be determined in the
`maximum_position` kernel function and returned. With this change there
is no need to handle unraveling of the position in the Dask graph. Only
the structured field need be selected, which is a constant time
operation. Thus simplifying the Dask graph generated by
`maximum_position` and improving the computation performance as well.

* Use N-D structured array in `minimum_position`

Rewrite `minimum_position` to use a structured `dtype` with a 1-D
field.  This allows the unraveled position to be determined in the
`minimum_position` kernel function and returned. With this change there
is no need to handle unraveling of the position in the Dask graph. Only
the structured field need be selected, which is a constant time
operation. Thus simplifying the Dask graph generated by
`minimum_position` and improving the computation performance as well.

* Use N-D structured array fields in `extrema`

Make use of N-D structured array fields in `extrema`'s kernel function
to pass back N-D coordinate positions. Avoids having an explicit
unraveling step added to the Dask graph after the `extrema`'s kernel
function.

* Drop `_unravel_index` from `ndmeasure._utils`

This function is no longer needed as unraveling of indices now occurs
within kernel functions that are returning position. Hence it is dropped
along with its tests.

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