ScalarQuantity now converts strings to floats (#934)

i.e. ```"2" | units.m``` is now equally valid as ```2 | units.m```.

src/amuse/units/quantities.py

@@ -283,6 +283,8 @@ def __init__(self, number, unit):
         # Quantity.__init__(self, unit)
         # commented out super call, this speeds thing up
         self.unit = unit
+        if isinstance(number, str):
+            number = float(number)
         if unit.dtype is None:
             self.number = number
         else: