WGL-93 data loading classes

doc/multiple_loading.md

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+Multiple data loader
+====================
+Metadata file
+-------------
+```json
+{
+  "files": ["file1", "file2"]
+}
+```
+Name convention
+---------------
+Every file contain N records. The records are continuously numbered form 0. The name of file is first_record_ID/N.
+
+For example:
+```
+filename    ID range
+0.csv       <0, N - 1>
+1.csv       <N, 2N - 1>
+2.csv       <2N, 3N - 1>
+...
+```
+```csv
+ID,x,y,accident_index,timestamp,accident_severity,road_type,speed_limit
+0,127.59738453,85.12878333,201243R249122,1356994800,2,6,30
+1,128.43117867,85.63623004,2012460222555,1356995160,3,6,60
+2,127.8970816,85.90564471,2012471300002,1356995340,2,3,30
+3,127.9295808,85.04589612,201201TB01286,1356995520,2,6,30
+4,126.33486436,82.66046783,201206N068460,1356996600,3,6,30
+5,128.03807716,85.07476281,201201KF69105,1356996600,3,6,30
+6,127.78275271,85.23497257,201201VK30371,1356996780,3,3,30
+7,126.86014507,82.51395481,2012130000038,1356996900,3,6,30
+8,128.06074311,85.06769119,201201JI40714,1356997500,3,6,30
+9,126.94752924,83.52010252,201230D002704,1356998160,2,6,30
+```

src/experimental/ClassicDataLoader.js

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+/**
+ * Classic data loader
+ * @param {string} datafile csv file with data
+ * @param callback call back function
+ * @constructor
+ */
+WGL.experimental.ClassicDataLoader = function (datafile, callback) {
+  this.load = function(){
+    d3.csv(datafile, function (err, data) {
+      callback(data);
+    });
+  };
+};

src/experimental/MultipleDataLoader.js

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+/**
+ * Data loader for multiple files
+ * @param {string} metafile metadata file
+ * @param callback call back function, one parameter for data
+ * @constructor
+ */
+WGL.experimental.MultipleDataLoader = function (metafile, callback) {
+  this.load = function(){
+    let patha = metafile.split("/");
+    let path = metafile.replace(patha[patha.length - 1], "");
+
+    d3.json(metafile, function (err, config) {
+      let q = queue();
+      config.files.forEach(function (data, i) {
+        q.defer(d3.csv, path+config.files[i]);
+      });
+      q.awaitAll(function (err, datas) {
+        callback(datas.flat());
+      });
+    });
+  };
+};