Use prop instead of naming layer name within Widget Class

[jsanch]

StyleToggle and Range (maybe there are others) widgets are now naming layer names wihtin the class instead of using the one passed in to the class via prop arguments.

Example of this in StyleToggle:

isthmus-react/src/components/widgets/StyleToggle.js

Line 34 in a67e5a4

this.props.layers.railaccidents.style.setContent(this.props.cartocss);

The fix should be to replace this.props.layers.railaccidents.style.setContent(this.props.cartocss) with this.props.layer.style.setContent(this.props.cartocss)

Example of this in Range:

isthmus-react/src/components/widgets/Range.js

Line 69 in a67e5a4

this.props.layers.railaccidents.source.addFilter(filter);