This commit ensures that we don't issue misleading warning about
output creation directives such as
BASIC
FREQ
RESTART
that could be, and typically are, put into keywords like RPTRST and
RPTSCHED. Previously, we would interpret these directives as names
of cell vectors and we would get very confusing diagnostic messages
of the form
Keyword 'BASIC' is unhandled for output to file
which suggests that 'BASIC' is unsupported despite the output system
supporting the directive for quite some time. While here, also
apply the same reasoning to the vectors we always output such as
SWAT, SGAS, or PRESSURE.
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