Fix: correct column index assertion in Matrix_Corner_Block and Matrix_Periodic_Banded #566

In src/matrix_tools/matrix_corner_block.cpp and src/matrix_tools/matrix_periodic_banded.cpp both get_element and set_element contain same assert twice assert(i < n); and not the check for j any. This maybe copy-paste typo accidentally.

double Matrix_Corner_Block::get_element(int const i, int const j) const
{
    assert(i >= 0);
    assert(i < n);
    assert(j >= 0);
    assert(i < n);   //This should be checked for j;
   ....
}

and

void Matrix_Corner_Block::set_element(int const i, int const j, double const a_ij)
{
   assert(i >= 0);
   assert(i < n);
   assert(j >= 0);
   assert(i < n);    //Same for this one;
  .....
}

double Matrix_Periodic_Banded::get_element(int const i, int j) const
{
    assert(i >= 0);
    assert(i < n);
    assert(j >= 0);
    assert(i < n);   //Same for this one;
   .....
}

void Matrix_Periodic_Banded::set_element(int const i, int j, double const a_ij)
{
    assert(i >= 0);
    assert(i < n);
    assert(j >= 0);
    assert(i < n); //Same for this one;
   .......
}

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