arrival model cleanup

Prosa currently contains specifications of the following arrival models:

1. periodic tasks: model/task/arrival/periodic.v
2. sporadic tasks: model/task/arrival/sporadic.v
3. arbitrary arrival curves: model/task/arrival/sporadic.v
4. request-bound functions (which, strictly speaking, is more than an arrival model): model/task/arrival/request_bound_functions.v

Additionally, we have the following "auto-conversions":

• (1)➔(2): analysis/facts/periodic/sporadic.v
• (3)➔(4): model/task/arrival/arrival_curve_to_rbf.v

I see two issues here:

• We are missing an auto-conversion (2)➔(3). This should be trivial, but for completeness we should provide it in Prosa.
  Question 1: Do we already have it in some to-be-cleaned up branch? @mmaida @sbozhko
• The auto-conversions are spread across different places and thus difficult to discover. Either they should all be in model/task/arrival/${foo}_to_${bar}.v or all in analysis/facts/.
  Question 2: Do you have a preference where it should go? @sophie @proux @mmaida @sbozhko

I'd like to sort these out in the coming days. Please comment.