move event.mcheader and/or replace it with a metadata dict

[kosack]

Right now there is a lot of MC header info in a Container (MCHeaderContainer) in the event data structure. Two things should be done:

1. move this outside of the event data structure, since it does not change per event (similar to the inst container, which will also move soon

2. decide whether it makes sense that this info is in a Container at all: Containers represent rows in an output table, so if there is no use-case for writing this as a row, we could just directly copy all header info in a simtel file into e.g. the event.mc.meta dictionary (the metadata for the event.mc table), which would be output as attributes or header keywords in the output file by a TableWriter subclass.

[kosack]

It was noted in #921 that there is a use case for (2) being a Container, in that we might merge multiple simtel files into one output file, in which case we'd want a table of these parameters (and also perhaps that means we need to store the mc run number with each event to be able to associate the two)

[maxnoe]

I think with the dl1 tool developement it is pretty clear that we want to have access to this kind of information in a container and that it makes sense that an event knows about its mc header, since we might work on an event stream from several runs.