Make Fact#time_stamp= a "set_once" setter

* can be set once (also later then initial creation)
* after that, cannot be set to a different value
* this implements a "form" of immutable behavior
Update comment about "fact stream"
Improve the setting of the Fact#time_stamp in Graph#<<

* allow pre-set time_stamp (don't touch it then)
* raise_error is new_time is _smaller_ then newest_time_stamp
* better precision (BigDecimal) for the 2 ns increase if
  new_time is equal to newest_time_stamp