negative population densities in cooling_function_wrathmall.py

[mherkazandjian]

The bug can be reproduced by running cooling_function_wrathmall.py in edaa3f9 and setting a breakpoint at https://github.com/mherkazandjian/frigus/blob/wrathmall_frigus/src/python/examples/cooling_function_wrathmall.py#L61