[Bugfix] Direction of the butterworth line filter

• libertem_holo.base.filters.butterworth_line() now always points from the center towards sb_position. Before, the direction was reversed when the slope of the line was between 0 and 1, so that in the aperture from libertem_holo.base.utils.HoloParams.from_hologram() the line went through the center of the sideband (and, in aperture_bf, through the central band). For sideband positions that were not affected, the aperture is unchanged.

• Sideband positions on the horizontal or vertical axis now get a line filter; before, the line was missing.

• butterworth_line() raises a ValueError if sb_position is at the center, instead of returning NaNs.