kymo: fix bug in Kymo.plot_with_channels()

[JoepVanlier]

Why this PR?
It seems that we did not consider the start time of the kymo versus the timestamp of the first data point when implementing Kymo.plot_with_channels().

f = lk.File("kymo.h5")
kymo = f.kymos["test_kymo"]

kymo.plot_with_channels(
    [f["Photon count"]["Red"].downsampled_by(100),
     f["Photon count"]["Red"].downsampled_over(kymo.line_timestamp_ranges())],
    adjustment=lk.ColorAdjustment([0, 0, 0], [95, 100, 100], "percentile"),
    titles=("Kymograph", "Downsampled 100", "Downsampled kymo")
)

plt.gcf().axes[0].get_images()[0].set_interpolation('none')
for i, ax in enumerate(plt.gcf().axes):
    ax.axvline(13.61, color="w", linewidth=2, alpha=0.5)
    ax.axvline(13.61, color="k", linewidth=1, alpha=0.5)

    ax.axvline(49.6, color="w", linewidth=2, alpha=0.5)
    ax.axvline(49.6, color="k", linewidth=1, alpha=0.5)
    ax.set_aspect("auto")

plt.xlim([49, 50])
plt.tight_layout()

Without:

With: