screenIsotopeData: Function to plot and screen stable isotope data with one or more baselines.
Description
This function receives a named list of vectors (isotopeData class object),
and plots a biplot with 2 sources and a consumer. The user can states whether
he/she wants a density function plotted above, to the right, at both sides or
does not want it to be plotted.
string representing whether the density function is plotted.
Accepted characters are "both" in which case will plot the density function
above and to the right, "right", "above" or "none".
consumer
string representing the consumer.
b1
string representing baseline 1.
b2
string representing baseline 2.
legend
coordinates representing where to locate the legend.