New `to_units` methods to allow changing units in a chain

@znichollscr suggested:

Currently, to convert the units you have to do `thing.Units = cf.Units("Tg")` and then you can carry on. Given how helpful chaining is, just adding a simple "to_unit" API would be great, so you could do `thing.to_unit("Tg").next_operation().follow_up()`.

PR to follow.