Simulation constructor start_datetime default argument potentially causes bugs

[jfeodor]

numerous/numerous/engine/simulation/simulation.py

Line 31 in 121cd00

start_datetime=datetime.now(), **kwargs):

I randomly stumbled upon this line in the the Simulation class constructor. The start_datetime argument has the default argument datetime.now(), which is evaluated when the class is defined, and not when it is instantiated. Alternatively the default argument could be None, and then redefined to datetime.now() inside the function body.