ENH: Have inferred_freq set offset. Have freq always try to infer.

I'm on the fence about this. Having a property with side effects doesn't quite sit right to me. But I think it kinda makes sense here. If it didn't set `freq`, then it really doesn't make sense for `freq` to exist. I'd just use `inferred_freq` all of the time.

Now calling `freq` always tries to infer the frequency. Why not? Seems sensible to me. `inferred_freq` is kind of redundant now, but I left it anyway. I was never really sure of the rationale for its existence.