Patterns using time_fn should be TimeDependent

[jlstevens]

There are several pattern generators that use time_fn independently when they should be (mix-in) subclasses of `TimeDependent``. The following classes should be defined as follows:

class Translator(PatternGenerator, numbergen.TimeDependent):
   ...

class Sweeper(PatternGenerator, numbergen.TimeDependent):
   ...

class Animation(SheetStack, numbergen.TimeDependent):
   ...

They should all use self._check_time_fn() in the constructor. This change will allow the global time_fn to be set in one place.

[jbednar]

I agree with this proposed change.