Added function to replace variables by their default values

[schymans]

Equations often contain variables with default values (e.g. physical constants) and variables without. If we substitute the dictionary with default values into an expression, e.g. demo_fall.xreplace(Variable.__defaults__), we obtain the RHS of the equation with inconsistent units, as the dictionary does not contain the units: 4.9*t**2
To simplify substitution of default values with their appropriate units into equations, I created the function replace_defaults() to variables/utils.py. Usage example:

>>> replace_defaults(demo_fall)
Eq(demo_d, 4.9*t**2*meter/second**2)

[schymans]

Alright, this did not work, as I tested it locally on sympy-1.1.2.dev0, but the current master of sympy has undergone a major refactoring of quantities and units. Need to decide whether to adapt to the new sympy master or back to sympy 1.1.1.