Fill y0 with zeroes if not of the correct shape

[schneiderfelipe]

It is a common use case, most of the concentrations in a complex reaction network will be zero. So assume them zero if some are missing.

https://github.com/geem-lab/overreact/blob/ae55264300b8d0e13626f8c6fb6000e16f48fa45/overreact/simulate.py#L58-L60