reshape state array for a multiple-state element after solving the roots

[gdarkwah]

I am just reopening a previous pull request that I mistakenly closed

For example, an element with 2 states will have two flux functions: [f1, f2] and two s_zeros: [S0_1, S0_2].
Let's say S0_1 = [0, 1, 2, 3, 4] and S0_2 = [5, 6, 7, 8, 9], the current implementation of line 170 will return a state_array of:

array([[1, 2],
       [3, 4],
       [5, 6],
       [7, 8]])

instead of:

array([[1, 5],
       [2, 6],
       [3, 7],
       [4, 8]])

This means that when you call get_states()[0] on the element, it will return the first two values in S0_1 instead of the first S0_1 and S0_2.

The new modification solves the issue. I encountered this while developing a multi-state element for my class project.