Remove tests which are testing numpy.matrix class

[wenzeslaus]

tests/test_new_and_modify_matrix.py::test_basic_raster_functions_with_matrix
  /.../pypops/tests/test_new_and_modify_matrix.py:7: PendingDeprecationWarning: the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.
    a = np.matrix(a, copy = False)
...

PendingDeprecationWarning: the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.