see #440: b1001_0 as monitor fmt and test

* adding test for monitor flag
* adding test description for monitor
* more helpfull error parsing for monitor
* changing specification of blockette and field:
  previously e.g. 1001->3 was used. However,
  tr.stats.mseed.1001->1 ist not valid python code.
  Changed this to b1001_3 in order to gain a valid python attribute name.
see #440, pure refactoring, better variable names