Add a function to convert a PrimitiveTree object into a mathematical expression.

[Rabbytr]

One major advantage of genetic programming is the ability to obtain specific tree-like expressions.
However, the current representation of PrimitiveTree still involves function calls:

add(mul(sub(sub(x0, x1), add(x2, x5)), truediv(add(x3, x1), add(x0, x2))), mul(add(sub(x4, x4), truediv(x5, x2)), add(add(x2, x0), sub(x3, x3))))

This representation may not be easy to understand.
Converting tree-like expressions into mathematical expressions can significantly facilitate human observation and analysis. The above example can be simplified to:

(x1 + x3)*(x0 - x1 - x2 - x5)/(x0 + x2) + x5*(x0 + x2)/x2

Although this may still be challenging to comprehend, it is indeed clearer compared to the previous representation, right?

In this commit, I added a member function to PrimitiveTree that allows it to convert itself into a mathematical expression string:

class PrimitiveTree(list):
  ...
  def expression(self, sym_mapping: Dict[str, Callable] = None, **kwargs) -> str:
  ...

Example:

>>> tree = gp.PrimitiveTree.from_string("min(sqrt(pow(x0, 2)), min(x1, add(x0, x0)))", pset=pset)
>>> func_sym_mapping = {
            'add': operator.add,
            'pow': operator.pow,
            'sqrt': sp.sqrt,
            'min': sp.Min
        }
>>> terminal_sym_mapping = {
            'x0': sp.Symbol('x0', negative=True),
            'x1': sp.Symbol('x0', positive=True)
        }
>>> tree.expression(func_sym_mapping, **terminal_sym_mapping)
        2*x0

[cxl0506]

This function is so practical! This is exactly what I need. Manually condensing long mathematical expressions is too boring.

Also, there's a clerical error， which "'x1': sp.Symbol('x0', positive=True)" should be "'x1': sp.Symbol('x1', positive=True)" .

terminal_sym_mapping can also be represented by :

terminal_sym_mapping = {
f'x{i}': sp.Symbol(f'x{i}', negative=True) for i in range(2)
}