Refactor RepeatedOperation initializers

[danthorpe]

At the moment, it's very tricky to subclass RepeatedOperation from outside the module.

• Remove convenience initializers - as these cannot be used by subclasses.
• Try to minimize the messy initialization code.
• Unify / Cleanup / Remove the Repeatable & RepeatedOperation stuff - I think this has got a bit confused since it was first written.

This is to address issues raised in #199.

[danthorpe]

@jshier - I've made a start on these changes. Currently everything is passing tests (CI is currently failing only because coverage has dropped).

Let me know what you think - hopefully should be much easier to subclass these.

[danthorpe]

This is all looking good now. I renamed RepeatedGenerator to RepeatableGenerator and made it public. This is because I cannot get around having the initializer for when T: Repeatable a designated initializer, because it's in a constrained extension.