refactor(code): Rename validity module

From strength of findings to the more general validity module, which can then
in turn contain the 'add_to_findings' function which unsurprisingly adds
validities to findings. Makes more sense to me.

src/model/validity.py

@@ -28,7 +28,7 @@ def _combined_validities(
     return r"\-"
 
 
-def add_validities(
+def add_to_findings(
     findings_df: DataFrame, studies_by_intervention: DataFrame
 ) -> DataFrame:
     valid_subset = (
@@ -36,7 +36,7 @@ def add_validities(
             ["internal_validity", "external_validity", "citation"]
         ]
         .fillna(1.0)
-        .drop_duplicates(subset=["citation"])
+        .drop_duplicates(subset=["citation"]) # type: ignore
         .sort_values("internal_validity")
     )