Add prototype API for FM EK80

[LOCEANlloydizard]

• Add public compute_Sv_f() and compute_TS_f() for the API
• Add _cal_complex_samples_f() prototype
• Expose frequency_resolution and range_step parameters for spectral calibration
• Return prototype Sv_f xarray output structure: (channel, ping_time, svf_range, frequency)

Refs: #1034

[codecov-commenter]

Codecov Report

❌ Patch coverage is 55.00000% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.56%. Comparing base (6cf6cee) to head (c6886e3).
⚠️ Report is 45 commits behind head on main.

Files with missing lines	Patch %	Lines
echopype/calibrate/calibrate_ek.py	35.29%	11 Missing ⚠️
echopype/calibrate/api.py	66.66%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1664      +/-   ##
==========================================
- Coverage   85.58%   85.56%   -0.02%     
==========================================
  Files          79       78       -1     
  Lines        6998     7074      +76     
==========================================
+ Hits         5989     6053      +64     
- Misses       1009     1021      +12     

Flag	Coverage Δ
integration	80.81% <55.00%> (+0.17%)	⬆️
unit	60.22% <55.00%> (-0.20%)	⬇️
unittests	85.46% <55.00%> (-0.02%)	⬇️

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[leewujung]

Hey @LOCEANlloydizard, I thought of a few other params after our discussion the other day so thought I'd lay them out and see what you think. Feel free to ping for more discussions!

def compute_Sv_spectrum(
    self,
    NFFT: int,
    range_step: float = 0.5,
    window: str = "hann",  # TODO: should this be default to None?
    frequency_resolution: Optional[float] = None,
) -> xr.Dataset:
    """Compute broadband EK80 Sv spectrum, or Sv(f).

    Parameters
    ----------
    NFFT : int
        Number of points in the FFT window for computing spectrum.
    range_step : float, default 0.5
        Range spacing in meters for the output Sv(f) window centres.
    window : str, default "hann"
        Type of window function to apply to each FFT segment.
        Options include "hann", "hamming", etc.
    frequency_resolution : optional[float], default None
        Frequency spacing in Hz for the output spectral grid.
        When this is provided, the output Sv(f) will be interpolated to this frequency grid.
        If None, the output Sv(f) will be on the original FFT frequency grid.

    Returns
    -------
    xr.Dataset
        A Dataset containing frequency-dependent volume backscattering strength.
    """


def compute_TS_spectrum(
    self,
    point_locations: xr.DataSet,
    NFFT: int,
    split_front: float = 0.25,
    window: str = "hann",
    frequency_resolution: Optional[float] = None,
) -> xr.Dataset:
    """Compute broadband EK80 Sv spectrum, or Sv(f).

    Parameters
    ----------
    point_locations : xr.DataSet
        Locations of the points for which to compute the spectrum.
        This is required to be an xarray.DataSet with dimensions ("target_id") and
        data variables "ping_time" and "depth" for each ``target_id``.
    NFFT : int
        Number of points in the FFT window for computing spectrum.
    split_front : float, default 0.25
        Each echo spectrum is computed from a segment of the complex echo signal.
        This parameter specifies how to identify the segment around the specified point location.
        For example, if split_front=0.25, then 25% the NFFT length will be before the point location,
        and the remaining 75% will be after the point location.
    window : str, default "hann"
        Type of window function to apply to each FFT segment.
        Options include "hann", "hamming", etc.
    frequency_resolution : optional[float], default None
        Frequency spacing in Hz for the output spectral grid.
        When this is provided, the output Sv(f) will be interpolated to this frequency grid.
        If None, the output Sv(f) will be on the original FFT frequency grid.

    Returns
    -------
    xr.Dataset
        A Dataset containing frequency-dependent point backscattering strength.
    """