Previously you passed a FieldSelection to Filter::Execute to specify
which fields to pass from input to output. There is no real reason for
this as other information about input and output fields are member
variables to Filter. This moves field selection as a member variable as
well. (This should also help confusion when updating old code to new to
prevent users from mistaking field passing with input fields.

Also added a few convenience constructors to FieldSelection so that you
can call Field::SetFieldsToPass() with just the string of what you want
to pass.