Plot: added custom plots for multipl. series
Plot: small code cleanup
Plot: added commentary
Plot: fixed gnuplot error messages

- errors occured when plotting distributions or nodevaluelists of
multiple series with differing amount of batches
- errors occured because gnuplot didnt get any data for the "line" that
was supposed to be in the plot, but actually not existing in the batch
- added a timestamp check so the "lines" wont be initialized in the plot
beforehand, thus gnuplot doesnt expect them and no error will be printed
- also removed the check for "null" batches in the addDataSequentially
method, no null batches should occur in the first place
Plot: added custom nodevaluelist and dist plots