Fix broken tree drop tip anlayses.

Due to the recent changes of the remote R analsyis refactoring,
a complete() signal needs to emitted at end of the
analysis excution to display the outputs on the data
panel.
Move common signal emit logic to the base class.

For easier mainteinance, move the complete() signal emit
logic to the base class voAnalysis.
ENH: Improve default parameter settings for drop tip analysis.

Make the default parameter settings of the Tree Drop Tip analsyis
to work with the sample data.
Improve the error reporting and documentations.