Fixed problems with Petrick's method

[Aniny21]

#3

The following errors occur in certain cases.

ValueError: min() arg is an empty sequence

Here is an example of the input that would cause the error.

(1)
mt: 0 1 2 5 6 7

(2)
mt: 1 3 4 5 6 7 8 9 10 11 12 14

(3)
mt: 1 2 3 5 8
dc: 13 21 34

There seems to be a bug in the implementation of Petrick's method, which is executed when len(chart) != 0.

This pull request removes the multiply() and mul() functions and adds the Petrick class.

In addition, it has been changed to display multiple solutions when there are multiple solutions.