Need warnings for improper use of polar boundaries about athena

[c-white]

Point 3 above has been changed as of 68a14ed. Now only blocks at the same radius need be at the same refinement level. This should make checking for consistency and even full compatibility with AMR much easier (using the arrays of PolarNeighborBlock structs that already cover all neighbors around the pole at the same radius).

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[felker]

Status summary of the above items as of 2018-12-30:

1. Check still missing.
2. Added by @tomidakn on 2016-09-05 in 7dec74c
3. SMR compatibility condition changed on 2017-11-16 in 68a14ed, but check still missing.
4. Polar + AMR still not prohibited.

So checks for points 1, 3, and 4 should be added when addressing #196. Sounds good @c-white?

By the way, I am making these checks trigger fatal errors, not warnings.