Naming conventions

@bbb @sbozhko @mlesourd @fradet @xiaojie @monin

We added an extra layer of hierarchy to start filling in the model in the restructuring folder.

We have chosen and applied some renaming rules for sections, cf. arrival_sequence.v and sporadic.v. Basically, we also use Valid* in the same way, same for respects*.

We have added some definitions of the sporadic task model. Our next step will be to add the TDMA scheduling policy to try and incorporate the RTA for TDMA.

This is still in progress, but comments are welcome!