Created by: sneaxiy

This PR provides a gentle and common way to read C++ global variables in Python. From now on, we can use fluid.core.globals() as a dict to get the C++ variables by its name.

For example, when we read a C++ variable named X, we can use:

x_value = fluid.core.globals()["X"]

When we write a C++ variable named X, we can use:

fluid.core.globals()["X"] = some_other_value

Also support the following methods:

# find whether "value_name" exists
"value_name" in fluid.core.globals() 

# return all variables names
fluid.core.globals().keys() 

# return "X" value if "X" exists, while return default value if "X" does not exist
fluid.core.globals().get("X", default=False)