Reduce maximum order for incoming external sky maps

We've been contacted by multiple experiments (Fermi, Swift, IPN) that would like to send us sky maps that are larger than we can send alerts with (>10MB in some cases).

We should provide a function that checks the length for incoming sky maps and will then reduce the maximum order to an acceptable amount by combining pixels in groups of four to one larger parent pixel, recursively.

PROBDENSITY
| ------ | ------ |              | ------ |
|  1e-5  |  1e-5  |      ====\   |  1e-5  |
|  1e-5  |  1e-5  |      ====/   | ------ |
| ------ | ------ |