Allow equations without explicit parameters

[tomicapretto]

If you pass a system like

du/dt = u - u * v
dv/dt = 0.3 * u - 0.3 * u * v

The app will not work and it will raise a message saying there are no parameters found in the system.
Currently, the correct way of specifying such a system is as follows:

du/dt = a * u - a * u * v
dv/dt = b * u - b * u * v

Where you input a = 1 and b = 0.3 once the app recognizes the parameters.

Ideally, the app should work with equations that include parameter values implicitly.