Changed default argument in observable.h: one can use intel compiler now

See merge request !1

up to now seeds were not saved to the dump file. Since the state of the
RNG was saved properly and the seed is only used in the initialization,
this likely did not affect the results.

the new layout is less likely to cause problems with dangling references

register_observable becomes optional,
the 'binsize' parameter is now required and becomes the default
binsize for new observables