Make URL styling method a function

As suggested by [codacy](https://app.codacy.com/manual/manycoding/arche/pullRequest?prid=4349920)

src/arche/tools/dataframe.py

@@ -3,13 +3,14 @@
 import pandas as pd
 
 
-class StylizedDataFrame(pd.DataFrame):
-    def _make_urls_clickable(self, val):
-        if isinstance(val, str) and re.search("^https?://", val):
-            return f'<a target="_blank" href="{val}">{val}</a>'
-        else:
-            return val
+def make_urls_clickable(val):
+    if isinstance(val, str) and re.search("^https?://", val):
+        return f'<a target="_blank" href="{val}">{val}</a>'
+    else:
+        return val
+
 
+class StylizedDataFrame(pd.DataFrame):
     def to_html(self, *args, **kwargs):
-        styler = self.style.format(self._make_urls_clickable)
+        styler = self.style.format(make_urls_clickable)
         return styler.render(*args, **kwargs)