Add CustomFuncFit into problem; Add related examples

tensorneat/common/activation/act_jnp.py

@@ -69,6 +69,10 @@ class Act:
         z = jnp.clip(z, -10, 10)
         return jnp.exp(z)
 
+    @staticmethod
+    def square(z):
+        return jnp.pow(z, 2)
+
     @staticmethod
     def abs(z):
         z = jnp.clip(z, -1, 1)