Add and expose time converters targeting stocks (#56)

### What problem do you want to solve?

This PR
- renames time converters for flow variables (e.g. earnings **per**
month) from `x_to_y` to `per_x_to_per_y` with x, y being our usual time
abbreviations.
- adds time converters for stock variables (e.g. number of unemployment
periods on the monthly **level**) named `x_to_y`
- exposes the stock time converters to make them importable via
`ttsim.interface_dag_elements`.

Author: MImmesberger

CHANGES.md

@@ -6,6 +6,7 @@ releases are available on [Anaconda.org](https://anaconda.org/conda-forge/ttsim)
 
 ## v1.1.1 — unpublished
 
+- {gh}`56` Add and expose time converters targeting stocks.
 - {gh}`53` Raise error if scalar is passed to ColumnFunction expecting array input.
   ({ghuser}`MImmesberger`)
 - {gh}`52` Fix fail_if checks: input data tree was too greedy and environment had an

src/ttsim/__init__.py

@@ -11,7 +11,7 @@
     version = "unknown"
     version_tuple = ("unknown", "unknown", "unknown")
 
-from ttsim import plot
+from ttsim import plot, unit_converters
 from ttsim.entry_point import main
 from ttsim.interface_dag_elements.shared import (
     copy_environment,
@@ -45,6 +45,7 @@
     SpecializedEnvironmentForPlottingAndTemplates
 )
 TTTargets = TTTargets
+unit_converters = unit_converters
 
 __all__ = [
     "InputData",
@@ -62,6 +63,7 @@
     "main",
     "merge_trees",
     "plot",
+    "unit_converters",
     "upsert_tree",
     "version",
     "version_tuple",