Fixing refinement max level in merger inputs files

[khanakbhargava]

This fixes the max_level error that I got when running in debug mode since the maximum allowed is 15.

[zingale]

looking at where MAX_LEV is used, I think that the assert may actually be wrong. I think that MAX_LEV is the number of levels, 0-based, so in Castro.cpp, the check that:

BL_ASSERT(max_level <= MAX_LEV);

should actually be:

BL_ASSERT(max_level < MAX_LEV);

this is what we assert on in Gravity.cpp and Diffusion.cpp