New functions for series in dataset in charts

[malloc82]

New series function:

• update-series: update a series' data points with either append or overwrite. This should work on both scatter plots and line plots. Also since time series is a line plot (xy-plot), it should work for that too. I only tests for the scatter plot and line plot
• remove-series: remove a series with a given label name in all datasets
• get-series-by-label: search through all datasets in order, return the series that has given label. If not found, return nil. Note that, there is a get-series function already, so I named this as get-series-by-label. But I do feel the existing get-series behavior is not correct, since it returns all series of given datasets. I suggest that one should be renamed as get-all-series, and this get-series-by-label should be named as get-series. Of course, there is backward compatibility issue to consider, so maybe it's good to rename them for 2.0 release?
• list-series: list all the series in a has-map format with keys are dataset IDs.
• clear-chart: remove all the series from all the dataset. The chart will be empty after this.

Corrected function behaviors:

• has-series?: previously can only tell if the primary dataset has given label. But since in one chart, there could be more than one datasets, this function should search through all datasets, and return true or false based on result.

Also added some tests for this these functions as well.