High-dimensional analysis ``brainpy.analysis.FixedPointFinder`` should be generalized to mutiple variables

[chaoming0625]

Please:

• Check for duplicate requests.
• Describe your goal, and if possible provide a code snippet with a motivating example.

Currenty, brainpy.analysis.FixedPointFinder only receives a function f_cell which only get one argument. This restricts the method applied to more genral cases. Becauce multiple variables usually exist in a dynamical model. Therefore, brainpy.analysis.FixedPointFinder should be generialized to multiple variables.

[chaoming0625]

Has been solved in #230