Create simple(r) event-monitor with NeXus capability

We should from these bits and pieces:

* `Event_monitor_simple`
* `Flex_monitor_*`
* The save-section of `Monitor_nD` + `mcdetector_out_list()` / `mcdetector_out_2D_list`  -> `mcdetector_out_2D_ascii()` or `mcdetector_out_2D_nexus`

be able to create a simple (non-propagation) event monitor.