Return None in functions that modify quantities in place

- Quantity.ito
- Quantity.ito_base_units

See:

https://docs.python.org/3.3/tutorial/datastructures.html#id3

BACKWARDS INCOMPATIBLE CHANGE
Quantity unit conversion methods are now independent from their in-place relatives.

Previously, `Quantity.to` was calling `Quantity.ito` after a copy operation. In the
same way, `Quantity.to_base_units` was calling `Quantity.ito_base_units`. Now, each
function has it's independent code path which call the right math (in-place or not
in-place) function.