Create version of LayerControlGroup for mutually exclusive layers.

[jamesscottbrown]

It would be useful for the LayerControlGroup to accept a Boolean mutuallyExcluive prop.

If true, then the controls for each layer would act like radio buttons, rather than checkboxes: enabling a layer would disable any layer that was previously selected.

The functionality of only allwoing a ingle layer from a group to be selected is used in several of our maps, particuarly for layers such as choropleths that cover the whole area, so that only one would be visible at once.