[Units] Add Cython interface for some functions

These functions are chosen for their potential utility. Setting units is
not enabled by this interface because it is meant to be handled
internally. However, these methods can help to determine the unit
factors for reaction rates.

include/cantera/base/Units.h

@@ -62,7 +62,7 @@ class Units
     bool operator==(const Units& other) const;
 
     //! Return dimension of primary unit component
-    //! ("mass", "length", "time", "temperature", "current" or "quantity")
+    //! ("mass", "length", "time", "temperature", "current", or "quantity")
     double dimension(const std::string& primary) const;
 
 private:

interfaces/cython/cantera/_cantera.pxd

@@ -100,6 +100,7 @@ cdef extern from "cantera/base/Units.h" namespace "Cantera":
         CxxUnits(string, cbool) except +translate_exception
         string str()
         double factor()
+        double dimension(string) except +translate_exception
 
     cdef cppclass CxxUnitSystem "Cantera::UnitSystem":
         CxxUnitSystem()

interfaces/cython/cantera/units.pyx

@@ -1,5 +1,6 @@
 # This file is part of Cantera. See License.txt in the top-level directory or
 # at https://cantera.org/license.txt for license and copyright information.
+from typing import Dict
 
 cdef class Units:
     """
@@ -21,6 +22,29 @@ cdef class Units:
     def __repr__(self):
         return f"<Units({pystr(self.units.str())}) at {id(self):0x}>"
 
+    def __str__(self):
+        return pystr(self.units.str())
+
+    def dimension(self, primary):
+        return self.units.dimension(stringify(primary))
+
+    @property
+    def dimensions(self) -> Dict[str, str]:
+        """A dictionary of the primary unit components to their dimensions.
+
+        .. versionadded: 3.0
+        """
+        dimensions = ("mass", "length", "time", "temperature", "current", "quantity")
+        return {d: self.dimension(d) for d in dimensions}
+
+    @property
+    def factor(self) -> float:
+        """The factor required to convert from this unit to Cantera's base units.
+
+        .. versionadded: 3.0
+        """
+        return self.units.factor()
+
     @staticmethod
     cdef copy(CxxUnits other):
         """Copy a C++ Units object to a Python object."""