Add Iteration-related Counter::Flags. Fixes #618 (#621)

Inspired by these [two](https://github.com/darktable-org/rawspeed/commit/a1ebe07bea5738f8607b48a7596c172be249590e) [bugs](https://github.com/darktable-org/rawspeed/commit/0891555be56b24f9f4af716604cedfa0da1efc6b) in my code due to the lack of those i have found fixed in my code:
* `kIsIterationInvariant` - `* state.iterations()`
  The value is constant for every iteration, and needs to be **multiplied** by the iteration count.
* `kAvgIterations` - `/ state.iterations()`
  The is global over all the iterations, and needs to be **divided** by the iteration count.

They play nice with `kIsRate`:
* `kIsIterationInvariantRate`
* `kAvgIterationsRate`.

I'm not sure how  meaningful they are when combined with `kAvgThreads`.
I guess the `kIsThreadInvariant` can be added, too, for symmetry with `kAvgThreads`.