add universal input control for all dataset variables

[alee]

In the new UI after selecting a dataset we should display a variable input control on all pages that allow the user to select the variable layer being displayed on the map or being rendered out with the timeseries graph

[alee]

This is implemented though we need to continue to refine the UI. Suggestions from the last team meeting:

• Restyle the variable input control and make it more clear that it is where you can change the variables. Add Variable: to variable input select dropdown on the bottom right of the Map
• When there is only one variable layer to be displayed, select it by default (as it appears to be working now) and set input variable in controls #55
• In contrast, when there is more than one variable layer available add a "select a variable" option to the options and do not select the first variable by default