revisit the structure of the package

[vtavana]

In this PR, the structure of the package is updated to facilitate future development.
The following changes are made:

1. A new file named _mkl_fft.py is created, and the exposed functions from the package are placed in this file (moved from _pydfti.pyx to this file).
2. Most of the remaining Pythonic functions from _pydfti.pyx are transferred to _fft_util.py.
3. _numpy_fft.py, _scipy_fft.py, _scipy_fftpack.py and _float_util.py are moved to the interfaces directory.

Note that the content of the functions and their functionality is not modified in this PR.

[Copilot]

Pull Request Overview

This PR reorganizes the package structure to prepare for future development without modifying the numerical functionality. Key changes include:

• Creating a new _mkl_fft.py file to expose public FFT functions.
• Moving functions from _pydft.pyx to _fft_util.py and shifting FFT interface code into the interfaces directory.
• Reorganizing imports and removing the obsolete _numpy_fft.py file.

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
mkl_fft/tests/test_fftnd.py	Updated tests to reference the new FFT interface at mkl_fft/interfaces/numpy_fft.py
mkl_fft/interfaces/scipy_fftpack.py	Changed the import style to align with the new package structure
mkl_fft/interfaces/numpy_fft.py	Moved FFT-related function definitions from _numpy_fft to the interfaces module
mkl_fft/interfaces/_float_utils.py	Adjusted docstrings to reflect the new module names
mkl_fft/_numpy_fft.py	Removed in favor of the new _mkl_fft.py file
mkl_fft/_mkl_fft.py	New module containing the core FFT function implementations
mkl_fft/init.py	Updated to expose FFT functions from the new _mkl_fft.py and re-export legacy functions

Comments suppressed due to low confidence (2)

mkl_fft/interfaces/scipy_fftpack.py:27

• [nitpick] Consider using a relative import (e.g., 'from .. import mkl_fft') instead of an absolute import for consistency within the package structure.

import mkl_fft

mkl_fft/init.py:42

• [nitpick] Since the majority of FFT functions are now exposed through _mkl_fft, consider consolidating irfftpack and rfftpack within that module for a more consistent public API.

from ._pydfti import irfftpack, rfftpack  # pylint: disable=no-name-in-module

[antonwolfy]

Thank you @vtavana