fix: issue#1 - Long-term ODE simulation shows negative number of cases

[lisphilar]

I think this was caused by non-dimensional ODE.
However, to keep development speed, I will close this issue with a minor change of codes.

It is necessary to revise the codes of classes to simulate ODE models and to perform dimensionalized numerical simulations later.

Advantage of non-dimensional system:

• we can estimate the parameter values efficiently and compare the parameter values
• we can compare the parameter values with that of the other countries easily

Dis-advantage of non-dimensional system (may be root cause of issue#1):

• substraction in diferencial equations reduces the accuracy of numerical simulation
• it is difficult to determine the minimum value of dydt
• dimensionalization of simulated values increases error with actual values