Added new event field Trigger that represents the accumulated analogue t...

This patch adds a new event field for neuromag machines. Instead of using the provided accumulated digital trigger channel, it calculates its own value using the individual binary channels. This is safer as the builtin methods sometimes fails when two binary channels have a slight delay.
The commit only adds code and leaves the rest untouched.
See also: http://bugzilla.fcdonders.nl/show_bug.cgi?id=1867