Divide by zero crash

[gdevenyi]

Documenting:

The root cause of the bug is that the if conditions in lines 61:78 in the cythonLaplaceStep function are sometimes not satisfied, and since counter starts at 0.0, and never increments, line tmpvalue = tmpvalue / counter results in a divide by zero error which crashes the program. For now I’ve wrapped the division in a check for if the counter is >0, but I need to check if perhaps there should be an else clause that would be more appropriate. The solution for the non-crashing left surface seem to be identical, so this looks okay.

[gdevenyi]

https://github.com/Mouse-Imaging-Centre/minclaplace/blob/master/cython_laplace.pyx#L61-L78