test(io): add csv, format and table tests

test/table_test.dart

@@ -0,0 +1,76 @@
+import 'package:cellar/cellar.dart';
+import 'package:test/test.dart';
+
+double num(CellValue value) => (value as NumberValue).value;
+
+Sheet salesSheet() {
+  final sheet = Sheet();
+  final rows = [
+    ['Region', 'Product', 'Amount'],
+    ['North', 'A', '100'],
+    ['South', 'A', '150'],
+    ['North', 'B', '200'],
+    ['South', 'B', '50'],
+    ['North', 'A', '75'],
+  ];
+  for (var r = 0; r < rows.length; r++) {
+    for (var c = 0; c < rows[r].length; c++) {
+      sheet.setCell(CellAddress(c, r), rows[r][c]);
+    }
+  }
+  return sheet;
+}
+
+void main() {
+  group('data table', () {
+    test('reads a header row from a sheet', () {
+      final table = DataTable.fromSheet(salesSheet());
+      expect(table.headers, ['Region', 'Product', 'Amount']);
+      expect(table.rowCount, 5);
+    });
+
+    test('sortBy descending', () {
+      final sorted = DataTable.fromSheet(salesSheet()).sortBy('Amount', ascending: false);
+      expect(num(sorted.cell(0, 'Amount')), 200);
+      expect(num(sorted.cell(4, 'Amount')), 50);
+    });
+
+    test('filter by a column predicate', () {
+      final filtered = DataTable.fromSheet(salesSheet())
+          .where('Region', (value) => value.display() == 'North');
+      expect(filtered.rowCount, 3);
+    });
+
+    test('groupBy with sum and count aggregations', () {
+      final grouped = DataTable.fromSheet(salesSheet()).groupBy(
+        ['Region'],
+        const [
+          Aggregation('Amount', Aggregator.sum, 'Total'),
+          Aggregation('Amount', Aggregator.count, 'Lines'),
+        ],
+      );
+      expect(grouped.headers, ['Region', 'Total', 'Lines']);
+      expect(num(grouped.cell(0, 'Total')), 375);
+      expect(num(grouped.cell(0, 'Lines')), 3);
+      expect(num(grouped.cell(1, 'Total')), 200);
+    });
+
+    test('pivot on region by product', () {
+      final pivot = DataTable.fromSheet(salesSheet()).pivot(
+        rowKey: 'Region',
+        columnKey: 'Product',
+        valueColumn: 'Amount',
+      );
+      expect(pivot.rowLabels, ['North', 'South']);
+      expect(pivot.columnLabels, ['A', 'B']);
+      expect(num(pivot.at('North', 'A')), 175);
+      expect(num(pivot.at('South', 'B')), 50);
+    });
+
+    test('selectColumns projects a subset', () {
+      final projected = DataTable.fromSheet(salesSheet()).selectColumns(['Product', 'Amount']);
+      expect(projected.headers, ['Product', 'Amount']);
+      expect(projected.rows.first.length, 2);
+    });
+  });
+}