#3 Wind speed indicator 3 and 0 not handled

[Anonymous]

When a message is generated from reports with a mixture
of wind speeds, message prep should choose a default iw
and convert all to the same, 0 should be 1, 3 should be 4.

[Adrian Phillips]

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Note WMO::Reports handles _iw for land synops incorrectly as
_iw can be 0 :-
$self->{_iw} = '/' if ! $self->{_iw} || $self->{_iw} == -1;
this and the couple of lines before should use exists.
- convert_iw should handle SHIPs also and doesn't handle iw
0 or 3 either
- messprep could call convert_iw from 3 to 4 and 0 to 1 and
this could handle that change as well, and it should also
change all iws to the "default" iw or to the special iw if
given.