Implement characteristic polynomial

Implement characteristic polynomial using `berkowitz`'s algorithm. Only the coefficients of the characteristic polynomial are returned, not the characteristic polynomial itself.
In this case `B = [1, -a - d, a*d - b*c]`. I have added a test for this very same case [here](https://github.com/thilinarmtb/csympy/blob/char_poly/src/tests/matrix/test_matrix.cpp#L911)