infinite loop in network construction when downstream id of an outlet is >= 0

[thurber]

If an outlet grid cell has a nonnegative downstream id that doesn't exist, a user may expect it to be treated as an outlet; instead they get stuck in an infinite loop due to the interactions between:

• https://github.com/IMMM-SFA/mosartwmpy/blob/main/mosartwmpy/grid/grid.py#L157
• https://github.com/IMMM-SFA/mosartwmpy/blob/main/mosartwmpy/grid/grid.py#L198
• https://github.com/IMMM-SFA/mosartwmpy/blob/main/mosartwmpy/grid/grid.py#L214-L216

Fix should be any negative j breaking the while loop, but should consider also setting the i to an outlet in that case -- may be other subtle issues.