A Material Design radio button.
Used to select between a number of mutually exclusive values. When one radio button in a group is selected, the other radio buttons in the group cease to be selected. The values are of type
T, the type parameter of the Radio class. Enums are commonly used for this purpose.The radio button itself does not maintain any state. Instead, selecting the radio invokes the onChanged callback, passing value as a parameter. If groupValue and value match, this radio will be selected. Most widgets will respond to onChanged by calling State.setState to update the radio button's groupValue.