Unit registry preprocessor to attempt to fix percent unit issue in python3.8

Author: softwareengineerprogrammer

src/geophires_x/Units.py

@@ -11,6 +11,7 @@ def get_unit_registry():
     if _UREG is None:
         _UREG = pint.get_application_registry()
         _UREG.load_definitions(os.path.join(os.path.abspath(os.path.dirname(__file__)), 'GEOPHIRES3_newunits.txt'))
+        _UREG.preprocessors.append(lambda s: s.replace('%', 'percent'))
 
     return _UREG