Refactor simulations

We're currently rebuilding the simulation code.

- [x] prepare output tables (AdL)
- [x] store station observables (AdL)
- [x] store coincidence (AdL)

In derived class:
- [x] generate shower parameters (DF)
- [x] simulate detector response (HM)
- [x] simulate trigger (HM)
- [x] GPS behavior (offset and standard deviation in timing) (HM)

Finally:
- [x] Fix ext_timestamp and (t1,t2,t3,t4) (AdL)
- [x] qsub to run simulations on Stoomboot
- [x] shower *energy* is currently stored as `shower_size`. Include both?
- [ ] test *all* code paths (use coverage.py) using unit tests (DF)
- [x] build a behavior test (this makes sure that the simulation output will remain unchanged while refactoring the code, unless we *want* it to change, of course).

Later:
- [ ] More realistic detector response (n1, n2, n3, n4, pulseheights, integrals) (HM)