Keep measure_obs in pl.utils only, not the pl namespace

Match the set_zero_in_cmap_to_transparent convention: measure_obs is a plain
public function in pl/utils.py, accessed via
`from spatialdata_plot.pl.utils import measure_obs` rather than promoted to
`sdp.pl.measure_obs`.

Author: timtreis

src/spatialdata_plot/pl/__init__.py

@@ -1,10 +1,8 @@
 from ._palette import make_palette, make_palette_from_data
 from .basic import PlotAccessor
-from .utils import measure_obs
 
 __all__ = [
     "PlotAccessor",
     "make_palette",
     "make_palette_from_data",
-    "measure_obs",
 ]