feat: add Quantity widget (using pint) (#126)

* wip

* simplified quantity widget

* fix example

* more docs

* add test

* update docs

* try to avoid overflow

* reduce again

tests/test_quantity.py

@@ -0,0 +1,31 @@
+from superqt import QQuantity
+
+
+def test_qquantity(qtbot):
+    w = QQuantity(1, "m")
+    qtbot.addWidget(w)
+
+    assert w.value() == 1 * w.unitRegistry().meter
+    assert w.magnitude() == 1
+    assert w.units() == w.unitRegistry().meter
+    assert w.text() == "1 meter"
+    w.setUnits("cm")
+    assert w.value() == 100 * w.unitRegistry().centimeter
+    assert w.magnitude() == 100
+    assert w.units() == w.unitRegistry().centimeter
+    assert w.text() == "100.0 centimeter"
+    w.setMagnitude(10)
+    assert w.value() == 10 * w.unitRegistry().centimeter
+    assert w.magnitude() == 10
+    assert w.units() == w.unitRegistry().centimeter
+    assert w.text() == "10 centimeter"
+    w.setValue(1 * w.unitRegistry().meter)
+    assert w.value() == 1 * w.unitRegistry().meter
+    assert w.magnitude() == 1
+    assert w.units() == w.unitRegistry().meter
+    assert w.text() == "1 meter"
+
+    w.setUnits(None)
+    assert w.isDimensionless()
+    assert w.unitsComboBox().currentText() == "-----"
+    assert w.magnitude() == 1