Added "min" and "max" sympy mapping (#473)

* Added "min" and "max" sympy mapping

To overcome the sympy bug while using Min/Max operators as discussed under this issue: https://github.com/MilesCranmer/PySR/issues/183

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pysr/export_sympy.py

@@ -47,6 +47,8 @@ sympy_mappings = {
     "ceil": sympy.ceiling,
     "sign": sympy.sign,
     "gamma": sympy.gamma,
+    "max": lambda x, y: sympy.Piecewise((y, x < y), (x, True)),
+    "min": lambda x, y: sympy.Piecewise((x, x < y), (y, True)),
 }