Refactor helpers module into processing

This commit refactors the `helpers` module into a `processing` module with a more logical structure. The `sandbox_observable_helpers.py` file has been split into three new modules: `adjustments.py`, `combinations.py`, and `transforms.py`.

This change improves the maintainability and readability of the codebase.

Author: google-labs-jules[bot]

src/rtichoke/discrimination/gains.py

@@ -4,7 +4,7 @@
 
 from typing import Dict, List, Sequence, Union
 from plotly.graph_objs._figure import Figure
-from rtichoke.helpers.plotly_helper_functions import (
+from rtichoke.processing.plotly_helper_functions import (
     _create_rtichoke_plotly_curve_times,
     _create_rtichoke_plotly_curve_binary,
     _plot_rtichoke_curve_binary,

src/rtichoke/discrimination/lift.py

@@ -4,7 +4,7 @@
 
 from typing import Dict, List, Sequence, Union
 from plotly.graph_objs._figure import Figure
-from rtichoke.helpers.plotly_helper_functions import (
+from rtichoke.processing.plotly_helper_functions import (
     _create_rtichoke_plotly_curve_times,
     _create_rtichoke_plotly_curve_binary,
     _plot_rtichoke_curve_binary,

src/rtichoke/discrimination/precision_recall.py

@@ -4,7 +4,7 @@
 
 from typing import Dict, List, Sequence, Union
 from plotly.graph_objs._figure import Figure
-from rtichoke.helpers.plotly_helper_functions import (
+from rtichoke.processing.plotly_helper_functions import (
     _create_rtichoke_plotly_curve_times,
     _create_rtichoke_plotly_curve_binary,
     _plot_rtichoke_curve_binary,

src/rtichoke/discrimination/roc.py

@@ -4,7 +4,7 @@
 
 from typing import Dict, List, Union, Sequence
 from plotly.graph_objs._figure import Figure
-from rtichoke.helpers.plotly_helper_functions import (
+from rtichoke.processing.plotly_helper_functions import (
     _create_rtichoke_plotly_curve_times,
     _create_rtichoke_plotly_curve_binary,
     _plot_rtichoke_curve_binary,