Generic data loading

[curran]

We'd like to have a data loader function/pattern, so that the application is structured that it first loads all the data sources, and is able to handle different data types (csv, json, svg).

Related feature idea: get status for a progress bar and once all data soruces are loaded and the main code can be executed.

Ideas for implementation, from previous work by @matthewsmawfield :

var dataSources = [
{'name': 'world', 'source': 'scripts/geo/topojson/worldhistory.json'},
{'name': 'centroid', 'source': 'scripts/geo/topojson/centroid.csv'},
{'name': 'disputed_boundaries', 'source': 'scripts/geo/topojson/disputed_boundaries.json'},
{'name': 'disputed_boundaries_polygons', 'source': 'scripts/geo/topojson/disputed_boundaries_polygons.json'},
// {'name': 'poc', 'source': 'scripts/geo/data/poc.json'},
{'name': 'poc', 'source': 'scripts/geo/data/data.json'},
{'name': 'layout', 'source': 'images/layout.svg'},
];

	if(arguments.length == 2){

		var data = [];

		var runstr = 'queue()';	

		sources.forEach(function(d,i) {

			var noCache = '?_=' + new Date().getTime();
			var s = "foo";
			if(d.source.indexOf("arcgis/rest/services") > -1){
				noCache = '';
			};

			if(d.source.indexOf("popdata.unhcr.org") > -1){
				noCache = '';
			};

			var method = 'd3.json';

			if(d.source.indexOf(".json") > -1){ method = 'd3.json';};
			if(d.source.indexOf(".csv") > -1){ method = 'd3.csv';};
			if(d.source.indexOf(".svg") > -1){ method = 'd3.xml';};
			if(d.source.indexOf(".xls") > -1){ method = 'xlsReq';};
			if(d.source.indexOf(".xlsx") > -1){ method = 'xlsxReq';};

			if((sources.length-1)!=i){
				runstr += '.defer('+method+', "'+d.source+noCache+'")';
			} else {
				runstr += '.defer('+method+', "'+d.source+noCache+'").await(function(){for (i = 1; i < arguments.length; i++) { data[sources[i-1].name] = arguments[i];}; callback(data); });';
			}
		});

		eval(runstr);
	}

Related work: https://github.com/chiasm-project/chiasm-dataset-loader