Use of `+ ε` and `- ε` in aRSA instantiations

The following discussion from !375 (merged) should be addressed:

• @bbb started a discussion: (+3 comments)

  It's not a big deal, but slightly unfortunate that we're mixing + ε and - ε in the different cases, especially since the - ε is necessary only to cancel out a + ε in the underlying definition of ep_task_intf_interval. It would be more elegant if we could standardize on using only + ε here.

  (I realize the existing EDF code already mixes this, which isn't ideal either. CC: @sbozhko).