layers/layer groups support

[hotzeplotz]

refactor main loop around layer/layer groups data structure:

• retrieve list of layer groups and their layers (this includes layer and layer group metadata, dataset(s) and crossfilter object(s)) (see process layer and layer group data structures #31)
• process variables and data separately for each layer group: as one dataset and group of variables (by variable type) for intersection layer groups, as distinct datasets and groups of variables (by variable type) for union layer groups
• create menu items accordingly
• create and populate charts accordingly
• create and populate map layers and layer groups accordingly
• wire in D3 dispatches as needed