Discovery asterisk location and broken tests

Fixed a bunch of tests that were recently broken (except for the one mentioned in #235 because I am not sure what the problem with it is).

#190: Changed the MPCWriter to be responsible for handling the discovery asterisk.  It puts it on the first non-rejected observation that it flushes to disk (which will be the first in temporal order).  Removed discovery asterisk from the AcceptSourceDialog since it is no longer set until write-time.