Speed up notch filter spectrum fit

[cbrnr]

Adding @cache to _get_window_thresh results in a speedup of ~1.85x on my machine (15.48s vs. 8.37s) in the following benchmark:

from pathlib import Path
from time import perf_counter

import numpy as np

import mne
from mne.datasets import sample

mne.set_log_level("WARNING")

fname = Path(sample.data_path()) / "MEG" / "sample" / "sample_audvis_raw.fif"

raw = mne.io.read_raw_fif(fname, preload=True)
raw.crop(tmin=0, tmax=60)

start = perf_counter()
raw.notch_filter(freqs=np.arange(50, 251, 50), method="spectrum_fit")
elapsed = perf_counter() - start
print(f"{elapsed=:.3f}s")