Continue sol for issue #154: Removed indices property.

I removed the upper_index and lower_index properties I added few days ago because the user
shouldn't worry about this level of details. And the on_upper_boundary and on_lower_boundary flags are sufficient.
I've also removed the check whether on boundary from the application script sapps/euler_2d/shockbubble.py because the solver method apply_q_bcs does this already.