New python API for units handling.

Exposes all supported unit systems & types, and to_value()/to_string() functions.

Reviewed and enhanced by CampbellBarton, many thanks!

Differential Revision: https://developer.blender.org/D416

source/blender/python/generic/py_capi_utils.h

@@ -73,4 +73,6 @@ int       PyC_FlagSet_ValueFromID(PyC_FlagSet *item, const char *identifier, int
 int       PyC_FlagSet_ToBitfield(PyC_FlagSet *items, PyObject *value, int *r_value, const char *error_prefix);
 PyObject *PyC_FlagSet_FromBitfield(PyC_FlagSet *items, int flag);
 
+int PyC_RunString_AsNumber(const char *expr, double *value, const char *filename);
+
 #endif  /* __PY_CAPI_UTILS_H__ */