…od code

The fact that the 2D FFT used in `paganin_filter_tomopy()` is C2C, in
conjunction with the use of `overwrite_x=True`, means that the result is
able to be written into the input array. Thus, no new array is created
to hold the result of the 2D FFT.

Also move variable in final sum to match the order in which the
allocations appear in the method.

The padded `float32` array is cast to `complex64`, and the variable
originally assigned to the `float32` version is reassigned to the
`complex64` version.

The reassigned variable was the only thing referring to the `float32`
array, so the ref count of it drops to 0 and is deallocated.

The padded `float32` array gets deallocated early enough in the method
that it doesn't contribute to the *peak* memory usage of the paganin
filter method, which was verified by checking:
- the output of `LineProfileHook` when running the method
- the allocations and deallocations reported by the custom
  `MaxMemoryHook` in the tests during the method's execution

The method's code defines the reciprocal grid based on the padded
projections `complex64` array shape. However, the memory estimator was
incorrectly calculating the number of bytes the reciprocal grid takes
using the unpadded `float32` array shape.